Hi, idk such a soft but if you're on Windows 7 you can use bitlocker (i think right click on USB drive to get it work). Thus you'll be asked for a pass. each time you plug it on a pc.
I'm not looking to lock the USB drive itself, but folders on it. Say i have a folder named "Pr3acher's nude pics" and i just wanna put a password on that folder only.
Worst comes to worst, you could always use 7zip/winrar etc. to make an archive and put a password on it.
Just protect the whole USB drive with an password. That would require to key in the password if you like to use anything from that drive oncefor the timke as long as that USB Drive is inserted. Using 7zip , rar or whatsoever compression and archiving tool would mean that you've every time, you like to use a file, uncompress that archive and re-compress it again after done your work! That would need quite some time and would be boring within a few days!