I've been reading a lot of user reviews of various portable usb powered drives and I've noticed that there seem to be way more reliability issues with these usb drives than there are about internal drives in general.
I wonder how many of these problems could be attributed to inadequacy of USB ports to supply enough current for the drives. At best I'll bet that most are marginally supplied and some are under supplied. For example, I just read that WD MyPassport Elite drives require at least 650ma (reportedly as per a WD company tech). USB ports typically only supply 500ma. Some drives even require the use of two ports, but most do not.
Some come with very short USB cables, apparently to minimize resistance loss, maybe also to maximize data xfer speed. I've heard of a few people trying to use unpowered hubs also.
I agree with the poster. If you look at the thousands of user reviews for most of the portable hard drives you will see hundreds of 1 star ratings for mostly issues with the drive "stopped working" and "no longer recognized". These comments are from numerous users who have only lightly used the drive. It appears their reliability is nowhere near that of an internal drive. When you read case after case of users who can't get the drive to be recognized any more, that sounds like a problem that is almost exclusive to external drives. Suspecting power as the problem sounds logical.