Many portable drives come with backup software installed. It's possible that this software lives on a separate partition that the HomeWorx doesn't recognize. On your computer (assuming Windows PC) go to Control Panel > Administrative Tools > Computer Management > Disk Management. Plug in your USB drive and see what Disk gets added. If there is more than one partition you will need to delete all but one partition then expand the remaining one to the full size of the drive.
Warning - Make sure you are selecting the correct disk. All data will be erased from this drive, so be very careful!!!Once this is done, do a full format of the drive.
You may want to try some partition manager freeware available here:
AOMEI Partition Assistant