These external/portable hard drives are becoming a big problem, people are relying on them too much and they tend to be more fragile. The biggest problem I see would be plugging a drive with my whole line on it into someone elses Windows PC and getting it messed up from a virus/or OS problem.
Those can be used for backups but you really should have backups on a different type of media that the primary.
I'd much rather take my laptop so I don't have to worry about finding someplace that I can feel safe plugging my drive into. I know my laptop is going to be safe.