Heya,
Drives will last longer than you will live, if stored well. The problem however is that the technology to use those drives will not last that long. By the time you really need to access that data, it's either useless data, or you have to find a dinosaur to access it (which itself may pose problems). IDE is dieing. SATA is pretty much the standard. What if your computer's IDE channels die? They are more likely to die before those drives in this `hypothetical' situation anyways. How do you get new motherboard/controller to read those IDE drives if they're not even made anymore? Or what if the cost is prohibitive? Old tech doesn't get cheaper--it gets more expensive, because demand is gone and supply is halted. In 10 years, buying an IDE controller will be either near impossible to find, or ridiculously priced. To do what? Retrieve a few gigs of data from a decade ago? Data that probably doesn't matter? I make that statement because you're clearly not backing up a bank--you wouldn't be posting here if that was your actual function.
Backing up to drives is a bad idea. Too many things go wrong. Heads crash. Parts decay over time. Various things effect it's ability to actually hold data properly. And again, the hardware to use those drives with fades away faster than the drives do.
If you are looking to backup, you're better off with TAPE backups, not HDD backups. They hold gigs of data as well. They're cheap. You can drop tapes in mud and wash it off and it still works perfectly fine. They're far better backups. The only thing they won't really survive is physical crushing (though even wrinkled and broken, it can be retrieved pretty often!) and won't survive fire (nothing survives fire). They survive water, humidity, etc though. Oh, they won't survive in bleach or sunlight (but again, nothing does, not even rocks). Tape is way smarter for backups. Alternatively, put things on optical Media if you're unsure. Bluray is here, 50gigs of storage per disc. And then there's the even more popular method, off-site internet based storage.
Overall, I seriously recommend you check out TAPE backups. Cheap, work well, and truly backup.
Very best,