From your comments, we can assume that your USB adapter, drive, the file system on the drive are all correct, right? But even so StartOS is still having trouble writing the backup. After confirming the above, the easiest way through this is going to be to delete that contents of that drive then start a fresh backup. Without the resulting error number, we can’t say exactly what the issue is, but starting fresh will just sidestep it.
I am getting the same type error, based on the solution , should you have two backup devices and alternate between the two , so you always have clean drive to backup too, and wiping the older backup drive when its time to do a backup. Is there any guidelines on how often to do backup
It’s advice that I give that I think could be really useful, a simple easy way to have two backups, a newer and an older, as well as to have a backup of your backup. Alternating between two backup targets should be easy to do for most people, it’s not a huge mental overhead.
As to how often you back up, this is very specific to the individual and their use of the server. Things like LND/CLN you’d want to back up each time you open a channel. If you have photos on Nextcloud of your kids maybe regularly?