Well, I have tried to make a persistent Backtrack 5 boot USB several times and it never works
Details ~ I have used Unebootin, Lili and Universal USB installer from a Windows environment and made both 32 and 64bit KDE versions on two separate 8GB sticks.
The persistence never works. I have tried allocating different size to the persistence, the USB is reported as made properly but when I try to boot, After selecting persistent from the Backtrack boot menu, it gets to the splash screen and stops. I have left it for a couple of hours a couple of times, but it never boots. I tried to type in the root and toor and startx blind but that doesn't work either. Ctrl-alt+f1/f2 sometimes switches the splash to a black screen ... sometimes nothing happens. I have to force a reboot.
Also worth a note is that if it is a persistent 64 KDE it won't boot in any other mode either, but the 32 KDE will boot into all the others except persistence.
The only way to get a viable working 64 KDE is to make one without the persistence option.