How do I back up my save files onto an external flash drive on Xbox One?

The small print at the bottom of the page that I linked for you mentions:

"USB flash drive is not a hard drive."

Xbox's connection with an external drive is read-only. You can only read files that are already on the drive after connecting. You can't save game files or anything else on the Xbox main drive onto an external hard drive.

May I suggest you back up those other files, format the flash drive to the FAT32 file format, put the update back onto the flash drive and try it again. You should only have the one folder on the flash drive.

I've managed to get mine running this week by moving all of the game files on to an external drive (flash). I'm guessing the issue was caused by moving my save games onto a flash to play at a friends house and then trying to copy it back onto my internal
storage. In the end I just copied everything onto the flash and am running it from there. I've had no problems downloading and installing since. Again, I've no real reason as to why this has worked but it has.

so with the preview update, that zapped my 360 drive. but i was not playing that many games on it anymore, most of my important saves were backed up to a flash drive. will have to download games again. tried to see if file recovery was possible, but the
files have been corrupted with the factory reset.

but i will be getting an external SSD drive for my one, seen the benchmark tests on a SSD hooked to a one and there are seconds saved with loading times.

can you not install discs onto externals? i'm like 90% that i've installed one of the gta5 discs onto one of my flash drives...unless i just installed to the hard drive then moved it to the flash.