ras125
Mount & unmount commands & USB detection are different in sme8.
I have it working OK on sme8, so read this
http://wiki.contribs.org/USBDisksespecially the bit re sme8.
Then undo (remove) all your settings re USB, & mount point etc, delete your backup job settings, and start again.
If still no go, then report back.
As part of the troubleshooting process you can see if your USB drive is properly detected by inserting the drive cable & then running
fdisk -l
then use the mount & umount commands providee in the USB howto to manually mount & unmount the drive.
If that works then backup should work.
If you cannot mount the drive manually then something is wrong or you are doing something wrong.