I got BT5 x64 Gnome working on a 32Gb SDHC card (yes, overkill, I know). I had to try a couple of times with Unetbootin but it finally took and did a HD install from there. You might want to try other USB installers such as LinuxLive (I don't recall the other utilities names, but there was one mentioned in another thread that actually lists BT5 as a supported distribution).
Good luck!


