[whpalmer4]

Depending on the size of your reference materials, you could store them in encrypted disk images in the cloud, though at that point you would have to exercise more care about not making changes on more than one machine at a time, since the whole disk image becomes an opaque blob. You do get the advantage that the files are accessible (and backed up, in the case of DropBox) which wouldn't necessarily be the case if something happens to your external drive.