Second that. Rather than just erase the disk, I suggest creating a new partition. In disk utility, click on partition, select 1 partition, choose a format (HFS+ if you're only using it on macs, or MS-FAT if you're using it on PC's as well) and click on partition.
This erases and re-writes the whole partition table, rather than just zeros out data and should bring all our space back.
Saying that, the Jump is quite an old series of drives, and USB drives will fail eventually. If you have to reformat the drive several times and you keep getting phantom data (lack of space, my terms) then it's probably time for a new drive.
Hope that helps