Late-2009 iMac suddenly won't boot
I was living out of the country off and on from October 2010 to April 2011, then left again from June 2011 till March 2015. So during the longer period, I wrapped up my iMac (Intel Core 2 Duo) and put it in my friend's apartment. Because of then switching to a laptop and not thinking about my iMac, I had no idea that a couple years ago there was an offer to replace faulty hard drives of my exact model (although I've long passed the deadline so I can't verify my serial number qualified).
It's been bothersome ever since returning in March. I immediately had to reformat the drive and copy over everything from my backup, which was surprising. I have always had two partitions, one for the system and all of my "work" files (like design files, writings, etc) and another for photos, videos, music. The computer ran OK until mid-June (although it froze a lot and required frequent restarts), when it suddenly wouldn't boot. I was able to boot from external, clone any new files I had made, but when I tried to do a disk repair & verify on the startup partition of the internal drive, then I received an error/warning and that partition disappeared from the Finder.
The first time this happened, I reformatted the whole drive. But only about a week and a half later, it crashed again. And then again. Now knowing the general behavior, I clone drive before verifying, because the drive then always disappears when verification is attempted. The last two times I only erased the system partition, repairing it while it's empty (but getting response that there's nothing wrong with the drive), and then re-copied files. Didn't touch the huge photo/video/music portion of the drive. But I could only boot from the internal drive for a short period til it would crash again.
Sorry this is such a long description. Point is, it's still not working, and while it's a nearly-6-years-old computer, I wasn't using it for several years so it shouldn't be overworked. For the third time since mid-June, I'm now running on the external drive. It's a bit confusing that most of the drive still seems to work, the files are all there, it just won't boot. I haven't tried to verify/repair the photo partition, though.
Any thoughts about this? I live in NYC and don't have a car, so getting to an Apple Store without knowing the reason is not convenient.
iMac (21.5-inch Late 2009), Mac OS X (10.6.8)