you shouldn't really even have to replace it. unless you get a dud, hard drives can last for a very long time. i have an old seagate .5GB hard drive from 1995 that still works. i obviously have no use for it but it works nonetheless.
Yea mid quarter is a real pain. I like it when my drive fail at the end of the quarter.
Now a days if I have data I just can't lose, it goes on an extra hard drive and dvd.
When I make a new system, I like to buy a new drive for it.
Invariably a drive will fail before the last backup was done and data gets lost.
Its wise to replace your hard drive when the warranty has expired. this is because even the manufacture will not trust the drive. when you stop caring about your hard drive you stop caring about your computer. remember that's one of the most vital parts of the computer.
Secondly why spend all the time it takes restoring a backup when you can clone the drive and keep the old one on a shelf for later projects that are not as important as your everyday computer...
Hard drives today sell for about $60 even at best buy. Its worth knowing your data is safe for just under $30 dollars a year.