I would copy the contents to a PC and reformat NTFS. The freetime units support NTFS and this is the file system used if you use the box to format a usb device.
I have had problems with the very small Sandisk USB drives going corrupt. I think they are on the limit of reliability anyway and I noticed they do get very hot in the back of my Humax. Luckily I was able to read from the stick but could no longer write to it. Got it replaced on warranty but the second one went the same way. I now use a more conventional much larger USB stick which has so far been 100% reliable.