Can't automatically save to USB with Simple Backup

I'm having trouble getting Simple Backup to automatically save backup files to a USB drive.

The 'configuration' page provided an option to save to 'disk' which is the USB drive, but when it actually made a backup, it apparently moved the 'disk' folder to the hard drive, and designated the USB drive as 'disk-1'! I then went back to the 'configuration' page, and disk-1 was provided as an option for saving, so I selected it. But when I checked to see that it was saving properly, the USB drive was still empty; the 'disk-1' folder had been moved to the hard drive, and the USB drive was designated as 'disk-2'! I repeated this process, and sure enough, the 'disk-2' folder was moved to the hard disk, and the USB drive was now designated 'disk-3'!

Throughout this process, Simple Backup never wrote a single file to the USB drive, except once when I started a manual backup. This at least got the job done, but I would like to have backups done automatically.

Has anybody else had this problem? Or does anyone have any ideas about how I could fix it? There doesn't seem to be anything about it on the 'Simple Backup' website, nor did a web search turn up anything relevant.

I'm using Simple Backup 0.10.4, on Trisquel Mini 4.5 Any help anyone could give me would be appreciated!
Chris Adams.