I had a similar problem, I unlocked my Titan using the SD upgrade bootloader method (which formatted my 2GB San Disk Card in the process). I cooked a custom Rom but it wouldn't boot past the splash screen. Since I was out of time to re-cook and see what was preventing the boot I flashed the latest Sprint ROM. I copied a few files via activesync and left the office.
At home I tried to restore the backup of my card and got stuck in a loop formatting FAT32 on the PC then the Titan formatting it to RAW. After searching I found this thread:
http://forum.xda-developers.com/arch.../t-273487.html
It suggested formatting the card in a digital camera. I did, and it worked like a charm! My card is now visable in the PC and PPC. Just thought I'd share the love, props to XDA.