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Disk formatting
Learn how ASUS routers can be used to directly repartition and format attached USB disks from the command line. Follow a step-by-step guide or use a simple interactive menu-based script to automatically format disks to ext2, ext3, ext4 filesystem. Disks larger than 2TB aren't supported due to partition table size limits.
- Limitations of managing disks directly on the router
- Automatically formatting disks with the AMTM script
- Wipe/Zero disk with dd to avoid errors caused by old partition tables
- Create a new partition table
- Create a new partition
- Format a partition to ext2, ext3, ext4
- Adjust filesystem features like disk labels and journaling
- Disk mounting and unmounting
- USB Storage Device cannot exceed 2TB disk capacity.
- Physical access to attach and detach disks to router USB port.
- Basic command line skills.
There are some compatibility issues with partition tables:
- MBR 'Master Boot Record' has best compatibility with ASUS-WRT
- MBR supports maximum disk capacity 2TB (entire disk)
- GPT 'GUID Partition Table' supports disks over 2TB
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GPT is unfortunately not supported by fdisk on ASUS-WRT so devices using GPT will appear locked in fdisk and may return error message similar to "Found valid GPT with protective MBR; using GPT". In this case the existing GPT must be erased without fdisk. One solution is to zero the disk. Another solution is the disk formatting feature in AMTM.
Quote: "Master Boot Record partition table limits the maximum size of the entire disk (not just a partition) to 2TB. If you have a disk larger than this you need to use a GUID partition table (GPT). Whilst personally I always use MBR if possible for compatibility reasons, we know that Asus officially supports disks of at least 4TB, ergo they must also support GPT. But here's the rub, the router's version of fdisk doesn't support GTP partition tables let alone have the option to create them. So owners of such devices ... will have to partition them with GTP on another device and then skip the whole of that step ..." -- ColinTaylor, [SOURCE]
There are a few problems you can run into when unmounting disks by command line:
You may be unable to unmount because the disk is being used by something.
admin@RT-AC86U:/tmp/home/root# umount -f /tmp/mnt/SANDISK/
umount: can't unmount /tmp/mnt/SANDISK: Device or resource busy
This problem is difficult to avoid and the best you can do is try to stop any processes/scripts that may be utilising the disk. If you encounter this problem there is no advice available on how or what processes to kill, so the best recommendation is to unmount with the web UI.
Quote: "Don't use -f. The device must be cleanly unmounted. If the user cannot unmount the device then they shouldn't proceed. There's no easy way to solve this. It's really up to the user to know what processes are currently using the device and to terminate them. The usual way of identifying such processes is with the fuser command. Unfortunately that isn't part of the normal firmware, although it is in entware." -- ColinTaylor
You may find duplicate devices get created.
- umount command does NOT automatically remove the device mount point
- removing mount point must be done manually
- failure to delete the mount point for unmounted devices can result in a confusing list of duplicate "ghost" devices and other problems. This problem can be avoided by remembering to manually remove the directories you disk was previously mounted as.
Quote: "When the router mounts a USB drive it creates a mount point (which is just a directory) with the appropriate name in /tmp/mnt. When you unmount that device using the GUI the router also deletes the mount point. If you unmount the device from the command line you will probably not think to also delete its mount point - that's the problem. If you now physically remove the USB device and then plug it back in the router will look in /tmp/mnt and see that there is already a mount point with the name it wants to use. To avoid mounting the USB drive over the top of (what it thinks is) another device it adds a suffix of (1) to the name it's going to use." -- ColinTaylor
There are problems that can arise if you don't zero your disk before trying to overwrite the disk with a new partition table using fdisk.
"Quote:... 6. Zero disk" should be marked as mandatory rather than optional. Yes it is dangerous, but I suppose the whole procedure could be called that. My concern is as I outlined in the link. Namely, if the device has previously been formatted with a partition table (unlike flash drives that usually are formatted without a partition table) as soon as you write the fdisk changes the router will look to see if there are any valid filesystems in the partitions. In this situation it's highly likely this will be the case so the router will mount them causing us problems in the subsequent steps.
Side note: When I was testing these scenarios for amtm I ended up in some bizarre situations, partly caused by the above. For example; first I partitioned and formatted my device as ext4. Next I used dd to wipe out just the first sector of the device, erasing the partition table. I then plugged the device into a Windows PC. Windows asks me whether I want to format it, which I do as FAT32. This puts a FAT32 header in sector 0. I remove the device from my PC and plug it into the router. The router auto-mounts it, so I unmount it. I then use fdisk again to create a new empty partition table and add one partition.
The thing to note here is that when fdisk creates the partition table it writes the relevant information at the end of sector 0. It leaves the rest of the sector unchanged! So...... If I plug this device into a Windows PC it thinks that it's formatted as FAT32 because there is a valid header in sector 0. If I plug the device into a Linux box it sees it as having a single ext4 partition (even though we never ran mke2fs after wiping and recreating the partition table)." -- ColinTaylor
There are two (2) methods of formatting disks attached to your router.
- Automatically with the AMTM - Asus Merlin Terminal Menu script.
- Manually by following this guide.
Beginners and experts can benefit from using the automatic script to save time and stop human error. Doing the task manually requires reading and understanding what you're doing.
amtm is a shortcut manager for popular scripts for wireless routers running on Asuswrt-Merlin firmware. It is intended to be a helper script, a convenient shortcut manager to install and manage scripts on your router.
The fd format disk feature was beta tested and quickly introduced in Update v1.6.
Visit the Developer's website and the Forum thread for information.
- The process erases all data and partitions on the device.
- Backup valuable data.
- Format disk with FAT32 filesystem before beginning to ensure it is clean and detectable by ASUS-WRT firmware.
- To be on the safe side, remove all other attached USB devices from router before continuing.
Consider checking the ASUS Plug-n-Share Disks Compatibility List
Plug your USB storage device into either the USB 2.0 or USB 3.0 port.
- USB 3.0 High Speed port: best for large capacity disks typically used for sharing large media files and video streaming.
- USB 2.0 Low Speed port: suitable for small capacity flash-drives typically used only for router scripts like Diversion, Skynet, Stubby etc.
Tip: Insufficient shielding on USB ports for some router models may cause WiFi interference on the 2.4ghz band. You may choose to enable the Reduce USB Interference option in the router Web UI.
Login to the router via ssh using your preferred client.
To enable SSH (LAN ONLY) go to Administration/System
Do not enable SSH for WAN or you will get hacked.
We must begin by checking the information for the attached disk.
Things to note before continuing:
- df command will NOT show unmounted disks.
- fdisk -l command will show both mounted and unmounted disks.
fdisk -- DOS partition maintenance program
Show the partition table for every disk, including unmounted disks. Include -l option to list:
fdisk -l
My example below shows fdisk output:
admin@RT-AC86U:/# fdisk -l
Disk /dev/sda: 15.3 GB, 15376318464 bytes
255 heads, 63 sectors/track, 1869 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sda1 1 1870 15014912 b Win95 FAT32
From my output we can see:
- My disk is /dev/sda
- It contains 1 partition, mounted as /dev/sda1
- Total disk storage capacity is 15.3GB
- Disk currently uses block size of 512 kb (the default)
- Disk considers 1 cylinder to be 16065 blocks of 512kb
- Therefore 1 cylinder equals 8225280 bytes
df -- display free disk space
Show human-readable filesystem usage statistics for all mounted disks:
df -h
My example below shows df output:
admin@RT-AC86U:/# df -h
Filesystem Size Used Available Use% Mounted on
ubi:rootfs_ubifs 77.2M 63.9M 13.2M 83% /
mtd:bootfs 4.4M 3.3M 1.1M 75% /bootfs
mtd:data 8.0M 576.0K 7.4M 7% /data
/dev/mtdblock8 48.0M 1.6M 46.4M 3% /jffs
/dev/sda1 14.3G 2.3M 14.3G 0% /tmp/mnt/SANDISK
From my output we can see:
- /dev/sda1 is mounted at directory /tmp/mnt/SANDISK
- sda1 currently uses the disk label SANDISK
TAKE NOTE OF YOUR OWN DEVICE DETAILS.
The information seen above will be used for all future examples.
Before we can begin zero'ing the disk, creating new partition tables and formatting we MUST properly unmount the device AND remove it's mount point directory.
mount command will list mounted devices:
mount
My example below shows mount output:
admin@RT-AC86U:/# mount
ubi:rootfs_ubifs on / type ubifs (ro,relatime)
devtmpfs on /dev type devtmpfs (rw,relatime,mode=0755)
proc on /proc type proc (rw,relatime)
tmpfs on /var type tmpfs (rw,noexec,relatime,size=420k)
sysfs on /sys type sysfs (rw,relatime)
debugfs on /sys/kernel/debug type debugfs (rw,relatime)
mtd:bootfs on /bootfs type jffs2 (ro,relatime)
devpts on /dev/pts type devpts (rw,relatime,mode=600)
mtd:data on /data type jffs2 (rw,relatime)
tmpfs on /tmp type tmpfs (rw,relatime)
/dev/mtdblock8 on /jffs type jffs2 (rw,noatime)
/dev/sda1 on /tmp/mnt/SANDISK type ext4 (rw,nodev,relatime)
From my output we can see:
- /dev/sda1 is mounted at directory /tmp/mnt/SANDISK
- sda1 currently uses the disk label SANDISK
- sda1 is formatted with type ex4 filesystem
Stop any processes/scripts that may be utilizing the disk before continuing.
Unmounting your device can be done in two (2) different ways; using the router web UI or using the command line. It is preferable to unmount from the webUI for reasons outlined in limitations section and then skip to the next step and zero your disk.
umount command unmounts filesystems.
My example device SANDISK can unmount with this command:
umount /tmp/mnt/SANDISK
If the disk was unmounted properly you should no longer see it listed:
mount
Remember there are different ways to check mount status:
- df command will NOT show unmounted disks.
- fdisk -l command will show both mounted and unmounted disks.
umount does NOT remove obsolete mount-points for unmounted devices, so we should do it manually.
My example output below lists the contents of the /tmp/mnt directory:
admin@RT-AC86U:/# ls -l tmp/mnt
drwxrwxrwx 5 admin root 4096 Jan 01 01:00 SANDISK
From my output we see my example device has an obsolete mount-point.
The obsolete mount point is removed with this command:
rmdir /tmp/mnt/SANDISK
If it was successfully removed then it won't be listed anymore in /tmp/mnt directory:
ls -l /tmp/mnt
Before we begin creating a new partition table it is prudent to properly erase the old partition table. We can achieve this best by overwriting the beginning of the disk with 0's using a utility called dd data duplicator.
Previously this step was listed as optional due to risk of data loss but further research and user feedback has shown it is essential to avoid errors when creating a new partition table. For a better explanation please read the limitations section.
dd is affectionately nicknamed DISK DESTROYER because people can accidentally erase the wrong disk and lose their valuable data. It happens when mistyping the command simply by reversing the source and target disk.
To be on the safe side, remove all other attached USB devices from router before continuing.
WARNING: RISK OF ACCIDENTAL DATA LOSS IF TYPED INCORRECTLY.
My example disk /dev/sda can be zero'd with this command:
dd if=/dev/zero of=/dev/sda count=16065 bs=512 && sync
You see we included two (2) options in the command: bs is block size in bytes and count is the number of blocks to write. Using the default block size of 512 bytes, multiplied by a block count of 16065 we can overwrite the first 8225280 bytes of the disk with zero's. This will completely erase the old partition table and the beginning of the new partition.
Sources:
fdisk command allows managing disk partitions. We must use it to create a new empty MDOS MBR partition table, which is not compatible with disks larger than 2TB.
My example device sda can be modified with this command:
fdisk /dev/sda
From interactive menu, do these commands in order:
m (or help) to see options:
m
p to see all existing partitions on disk (take note):
p
o to erase and create new disk partition table of type mdos MBR Master Boot Record:
o
p to see all partitions on disk (now we should see none):
p
n to begin making new partition:
n
...then p to make it a primary partition:
p
...then 1 (one) for its partition number:
1
...then accept default values for first and last cylinder
p to see all partitions on disk (now we should see 1 created of type ID 83 Linux):
p
w to write changes to disk:
w
mke2fs command is to create an ext2, ext3, ext4 filesystem, usually in a disk partition, where device is the special file corresponding to the device (e.g /dev/sdXX ).
A few command options:
-t : set file system type,
-L : set a new volume label,
-O : specify a feature to use
For the commands below my example device is sda1. Your device may differ.
To format as ext2 filesystem:
mke2fs -t ext2 /dev/sda1
or use the alias ASUS built-in:
mkfs.ext2 /dev/sda1
To format as ext3 filesystem:
mke2fs -t ext3 /dev/sda1
or use the alias:
mkfs.ext3 /dev/sda1
To format as ext4 with journaling off:
mke2fs -t ext4 -O ^has_journal /dev/sda1
To format as ext4 with journaling on:
mke2fs -t ext4 -O has_journal /dev/sda1
tune2fs command allows to adjust various filesystem parameters on ext2/ext3/ext4 filesystems.
To add or change disk label:
tune2fs -L "yourDiskLabel" /dev/sda1
To disable journaling on ext4:
tune2fs -O ^has_journal /dev/sda1
To enable journaling on ext4 omit the ^:
tune2fs -O has_journal /dev/sda1
To disable 64bit filesystem compatibility:
tune2fs -O ^64bit /dev/sda1
To enable 64bit filesystem compatibility omit the ^:
tune2fs -O 64bit /dev/sda1
Rebooting the router is highly recommended to remount a USB device. You may encounter errors with ASUS-WRT if you try mount manually instead of rebooting.
Reboot ASUS router with this command:
/sbin/reboot
If a reboot isn't possible then try manually mount the device. My example device sda1 with label SANDISK can be mounted with this command:
mount /dev/sda1 /tmp/mnt/SANDISK
The first version of this guide was posted to the SNB Forums and led to the development of the disk formatting AMTM script. The guide is now maintained here on the Merlin Wiki.
Special thanks to thelonelycoder and ColinTaylor