25 minuten - 1 uur
The iMac internal HDD was failing. Late 2009 iMac i5 quad-core.
Internal HDD was failing badly. In fact, I waited too long, and was about, oh, literally 1 day too late in fixing it. I had received my orders from ifixit and amazon two days prior, but was waiting until the weekend to repair, but at that point the HDD was locking up so much I couldn't manage a migration or anything to the new HDD, so I spent about a day and a half learning about every possible way that should, but did not work, and finally managed to work out a solution. The iMac is running like a champ again, and I am more technically competent. Bless the repair partition in Lion OSX: it worked with the faulty HDD for the 1.5 hours I needed it to, so the folks at Apple must have been smart enough to have the repair process use the HDD at a very stable mechanical level.
If your HDD begins to fail, replace it ASAP. It will probably last for a while after it starts clicking: mine lasted 2 months. It may last two weeks after Drive Genius tells you there is an irreparable mechanical issue with the HDD, and it may last a one week after Disk Utility tells you the same thing. If you gamble with the time you have left, and loose (like I did) you're likely setting yourself up for a more difficult recovery process. There's no point to waiting: if the HDD is failing, it's failing: i.e., it's inevitably going to require replacement. Don't bother waiting, you'll spend the same money replacing it sooner or later.
Also, please understand the difference in utility between migration assistance and time machine restore: I could have saved myself an additional 6 hours of restore time.
Last: be a do-it-yourselfer and have faith that you can succeed. Lord knows how much it would have cost to fix this via the Apple store (way out of warranty), but I'll guess about $500 for another 1TB HDD. I spent about: $120 on a 3TB replacement, $50 for the ifixit supplies (most of that was for the optical bay/HDD switch), and about $90 for a 2.5 inch 1TB HDD for the optical bay. I spent about $300 for an UPGRADE, that cost considerably less than a simple repair. I also had a good time and learned a lot, expletives notwithstanding.
Oh, and because I waited too long I did have to buy a wired keyboard from the Apple store, and that cost me $50 I didn't need to spend (going to get a mini soon, so no loss, really).
12.7 mm SATA Optical Bay SATA Hard Drive Enclosure
Heavy-Duty Suction Cups (Pair)