My first thought if this would happen to me would be to try the external drive out on a totally different computer say a friend or family members.
This is assuming your pictures were probably just copied from the computer or card and pasted on the external drive.
If the pictures look fine on theirs then you may have a software issue on your computer.

My other suggestion is never trust just one source for protecting your precious files. Any kind of storage device that can be written to is not guaranteed to protect your data. I know it's a pain in the rear but backing up your important data onto a quality CD or DVD and properly storing the disc is the best proven method for saving it. And it's the cheapest! I have some 12 year old cdr's that still read fine after all this time and weren't really stored properly.

There are other options such as setting up a RAID setup, Online storage, etc. but sometimes the simple solutions work best.

Also your problem almost sounds like the data may have gotten corrupt. I have seen similar issues "bars on pictures or part of them missing" when recovering files off a deleted flash card. I"m cross my fingers for you for luck as I hate to see someone loose photos especially if there of family.