It sounds to me like you had a specific problem with a file system on your
USB drive. When you say you couldn't see the device on XP, did you mean it
didn't recognize the drive at all or that nothing was on it?
I do the same thing ( transfer a FAT32 formatted USB drive between Vista and
XP ) and don't have any problems. Now granted I am fanatical about always
unmounting USB devices ( i.e. "Safely Remove Hardware" ). I almost never
remove any USB device without doing this. It could be you had a problem with
the file systems on your USB drive and Vista was a little smarter about
reading around it. The convert would have "fixed" the problem. If you
encounter this again, I suggest you run chkdsk on that drive which will also
fix the file system.
I don't see anything on the Web about gotchas between the two OS's either.
But it could be you did hit a particular bug and maybe my disk doesn't have
so many files that the "FAT32" file system would be tested more. I did
notice that the Vista format allows you to set a cluster size of 64K which
is not supported by XP. So maybe you encountered that as well.