What precise hard drive do I need to install in this iMac? I was just given an old iMac without a Hard Drive.

This is the model (image from the software Mactracker):



It only gives the following details about the hard drive:

320, 500, 750 GB, or 1 TB 7200-rpm

However, when I consulted ifixit to see how I could replace the hard drive, and clicked on the suggested replacement hard drive, some instructions said:


  Important: To ensure compatibility with the 2009 and 2010 21.5" or 27"
  iMac, the replacement hard drive must be of the same brand as the
  original. The model number of the hard drive does not matter as long
  as the brand matches. 2011 and up iMacs cannot be supported by us.


Which doesn't help since I have the 2008 model. I am also not sure whether a 2.5 or 3.5 hard drive is required. The "About this Mac" menus are not available since the computer doesn't power on. I don't know what the RAM is (2.8 or 3.06).

What is the exact reference for a compatible model for this iMac Early 2008?
I'm going to try to answer your question as best I can.


The drive is definitely a 3.5" SATA disk.
The notice doesn't apply to your model.
I'm not sure why the notice suggests to choose the same brand. For one, the drives are likely co-branded, with markings for both Apple and the company that made the drive. The companies that would have supplied the drives would be, AFAIK: Western Digital, Seagate and (possibly) Hitachi.
I'm not sure how similar you have to go in terms of storage. I'd guess you could probably use any SATA drive, but because of the temperature sensor, if I were you, I'd use one of the ones suggested in the guide, depending on your storage requirements (1-4TB).


TL;DR: If you're super worried about compatibility and need to get the machine working fast, disassemble it and look at the HDD to determine the brand, then order that brand. Otherwise, I would suggest you buy whichever of the listed HDDs strikes your fancy - I doubt that it really matters.