Windows 7: USB Drive - Can't See Folders/Files

Hope this aint too much babling but I just can't seem to figure this one out...
I have a Vista x64 machine where if I copy files to a USB drive then plug the USB into my Windows 7 x64 machine, the Windows 7 machine cannot see the folder/files.
If I go back to the Vista machine, move the folder into an existing folder (that Windows 7 can already see) it's all good, even if I copy the folder back out to USB root, it's still good. I got some kind of security issue somewhere? Any thoughts?
Happens vice versa too.
Also, an XP Machine can see the same as Windows 7 but not the ones Vista sees, a Vista x86 can't see the Vista x64 files either.
So it's definitely in the Vista x64 machine.
I'm so confused
Both x64 machines are Ultimate versions. My XP and other Vista Home are both 32bit. Not too sure what else to tell ya.
On the x64 machines the USB is NTFS mounted (I already tried changing that to a letter), on the other two machines the USB just grabs a drive letter.
I have no other sharing issues and have many USB drives plugged into the Vista x64 and all other machines see the files. Only lately do new files not appear.

do you remove it safely? so no immediate unplug? that way, files may get corrupted