DAppNode can automatically detect and mount external SSD drives connected via USB, thanks to a fork of a utility called usbmount. If you plugged your drive and you cannot see it in the list of available mountpoints, check the following points.
The most likely issue is you used a drive which is not formatted with a format natively compatible with DAppNode OS. We don't support partitions in NTFS (Windows) or APFS, HFS+, etc. (MacOS). The recommended filesystem format is ext4, but you can also use FAT32.
Disk not formatted
Make sure your disk is formatted with a valid partition table and partitions. Use a utility like gparted to accomplish that.
If you added a disk internally in your DAppNode, it's not going to be mounted automatically, and you will need to configure it manually adding an entry in
/etc/fstab. As a reference you can follow this guide