As I also work heavy on usb disks as well, as per my own statistics I also have to denunce to often encouter an overwhelming quantity of "disk busy errors" , but it seems to happen more often after dealt on disks by gui software like Thunar or Caja.
For what concern your script maybe you can find usefull the following hints: 1) an error handled ~ error prone unmounting proc: unmountEtc() { (set +e;trap 'umount -f "/etc"' 0 1 2 3 5 7 10 15;sleep 5) || return } unmountEtc I hope that is still running well as resue old of years code is a "true pain".. 2) Please note: if you backup to a not existing destination subfolder in your mounting point your script will always ganerate an error and stop. It is very different behavior if your script backup to the root of the mounting point, indeed.. Said that, you have eventually just to manage the error when the destinationsubfolder doesn't exist in the mounting point. ;o)