iMac Intel 21.5" EMC 2308 - what's the oem hard drive?

Apple uses a variety of different manfacturers. If it's made after 2009 the hard drives Apple uses have the temp sensor integrated into it. You would be best suited to having Apple or an AASP replace the drive or you can also contact OWC (www.macsales.com) and see what they recommend for your machine.

OK, then follow my advice.

Getting the correct drive is the tough part, contact OWC as I've instructed 2x already.

It would help to know your OS version but, here goes:

for OS 10.6.8 and older, find System Profiler in Applications > Utilities and launch it.

For OS 10.7 and later find "System Information" in the same path and launch it.

In the left-hand navigation pane, click "Serial ATA" or "SATA/SATA Express" depending on your OS version. The main pane will change to something like this:

Note the info I've outlined in red. In the image, "WDC" is a Western Digital drive. Post the code for your drive and we can deterrmine what brand you have.

Ah, it wasn't clear that the computer was not working.

You can try calling Apple but I suspect the chances of their having that information available is remote.

Seagate had some issues some time back that still have me a bit nervous. I've used nothing but WD for internal drive for a logn time with zero issues, although I would not buy one of their externals--their external enclosures are notoriously flaky.

See this:

http://www.apple.com/support/imac-harddrive/

It concerns failed Seagate drives.