Detailed help qubes backup on Verbatim USB hard drive

From the name, this qube looks like a disposable VM template and not a disposable VM. The latter should have names like DispXXXX. The former is used to create new disposable VM; if it’s compromised, all dispVMs will be compromized too.

This looks like a problem with the filesystem on your USB hard disk. Is it FAT32? See this: Backing Up Hangs at 4GB (USB Stick)