aside from making everything relative in terms of data paths etc. don't treat a usb drive like a regular drive especially if using any raster data. The reasons are somewhat long winded, but that has been my experience teaching in a large computer lab since their inception... they are good or backup and transporting but are not 'real' drives at some level and at $6 a pop for the lower end ones, they shouldn't be considered anything other than a backup media.