The main problem is that an External Hard Drive is going to be a... well... I won't post an inoffencive word, so I'll rephrase that to "It will be difficult."
Repairing it may not even be an option, as many technicians know very little about the relatively newer technology.
However, if you do HAVE to replace, I would consider an External USB 2.0 Hard Drive ENCLOSURE, rather then the External Hard Drive.
This nifty gadget allows you to plug in a Desktop Hard-Drive, of practically any size your computer can recognize, and turn it into a USB 2.0 Hard Drive.
The handy features of this include, first and formost, if the Hard Drive Enclosure's buttons stick, or refuse to work, simply replace it, and install the Hard Drive from the first enclosure into the replacement, therefor saving all data on that drive.
This is especially useful in older machines where there is only one or two low-capacity hard drives onboard((Windows 98 will not tolerate anything over 32 gigs, but you CAN partition the hard drive into several smaller hard drives, causing Windows to think there is more then one disk there.))
As to the External Hard Drive itself, if the buttons do not work, there may be something durastically wrong with the Hard Drive as well, as most External Hard Drives are rather well integrated, so I say, if you can take it out, do it. But be sure to run Disk Checker or Scan disk on it, once you plug it in through other means, to ensure it isn't corrupted, or partially damaged.
I should also point out the rather large price differences:
1) A 500 gig External Hard Drive costs upward of $150 easily
2) A USB 2.0 External Hard Drive Enclosure is rarely more then $100-110, while individual hard drives((which you can swap out as you go, with relative ease)) cost anywhere from $50-100 making them about the same.
However, you start noticing the price value when the 500 gig((or smaller)) hard drive fills up, because there isn't much you can do about it, except burn it all onto DVDs((defeating the purpose altogether)), or get another hard drive. Whereas, An External Hard Drive Enclosure can have more then one 500 gig hard drive, you just swap them out as you need them. It also has the handyness of not being limited to that one size. For example, if you buy a 100 Gig external Hard drive, you are limited to only having the 100 gigs. But if you buy an External Hard Drive Enclosure, and a 100 Gig drive, If it proves to be too little, you simply buy a larger hard drive. However, for lighter users, typically an 80-160 gig hard drive would suffice. ((Depends on your' music/video/games library. These three are notorious for being Hard Drive wh... er.... uh... let's rephrase... These three are well known to be rather space intensive.))
If you have the right External Hard Drive, and when you get the thing open, you find a standard Desktop Hard Drive, rather then some of the higher-classed brands, which integrate it in annoying manners, making it impossible to take apart, an External Hard Drive Enclosure would be your' ultimate answer, as it would take this hard drive, and turn it into a removable disk. Consider the possibillity, however, that you may lose that external drive. Some of the higher-classed companies don't like people doing this, and have designed removable hard drives that can't be transferred to other hard drive enclosures, so the user must spend hours upon hours trying to get it fixed, at exorbitant rates, from the manufacturer, bringing them more money.
If you have any more questions, such as what kind of stores carry these items, it's usually not future shop, best buy, or target, but your' average run of the middle small computer store, and they aren't that expensive about it, either. As a boon, they also often sell Desktop Hard Drives of various sizes at significantly cheaper prices then say... Future Shi... er... Shop.