I think I had a similar problem too, I had an X-Flash 32mb USB drive, it detects with the hacked version of Mega Xkey and the Xbox. The problem with the drive was that it had two partitions, and my PC could only see the first partition (512kb) unless I ran a program included inside the flash drive, that unlocked the other 31mb of the drive. So.... the Mega X-Key software somehow sees all 32 megs of the drive, so I'm able to copy the 007 save to the flash drive, but my xbox only sees the first 30 blocks (512k) of the save, so it says the 007 save is corrupted. And since I formatted the flash drive in my xbox, the program is gone, now I can only use it to transfer small saved games.
Unless you know someone with a modded xbox, I think your only choice is to buy a Mega X-Key. You'll be waiting for your order for a while, maybe around two weeks.