Your EdgeRouter's internal USB drive died, and you don't have a spare couple hours to figure out how TFTP works. Run this script instead on your nearest Linux machine and generate a .img that you can dd straight to a new USB drive.
If you want to create a USB boot drive directly, use
sudo ./mkeosimg ER-e100.v1.xxxxxxx.tar(you downloaded that file from the Ubiquiti site. Pick the latest system update for your router).
If you have a backup config file that you'd like to include, such as edgeos_routername_NNNNNNNN.tar.gz, just provide the path to it as the second parameter.
If you want to use more of the available space on a larger flash drive, you can enter the total image size in GB as the third parameter. If you don't want to provide a config file, but still want to have a larger image, use "-" (without the quotes) as the second parameter. If no drive size is specified, the script will default to 2 GB.
Download the resulting .img if you built this elsewhere (such as on a Google Compute Engine instance that you spun up just for this project). You should have gzipped it because that'll make it around 20x smaller.
sudo dd if=ER-e100.v1.xxxxxxx.img of=/dev/path/to/usb/drive bs=1M(adapt to your OS of choice) to write the image to your USB drive.
If you want, you can directly create a USB drive without creating an image file first. Use
./mkeosdriveinstead, and supply the target device as the first parameter. All other parameters are shifted by one position accordingly.
sudo ./mkeosdrive /dev/path/to/usb/drive ER-e100.v1.xxxxxxx.tar
Insert the new USB drive into your router and boot it up. If you included your backup config in the first step, then your router will start shuttling around packets on your network just like it used to; otherwise you'll have to set it up as a brand-new device.
Inspired by https://github.com/vyos/emrk/blob/master/bin/emrk-reinstall. Some parts of the code were stolen from there as well.
Beware! There are lots of things out there that call themselves "EMRK" and succeed to varying degrees in helping with EdgeRouter unbricking.