I couldn't find much definitive information regarding changing the HDD for another HDD in a Late 2012 27" iMac. There are quite a few threads regarding changing the HDD for an SSD but none that I could see for a replacement HDD. I had a 1.25 TB Fusion Drive (which I'd created myself by adding a genuine Apple 250 GB blade SSD to my existing stock 1 TB HDD) but was fast running out of space. I had been using an external 1 TB HDD too but found it to be an inelegant solution - also on occasion it would disappear from view in Finder and need re-plugging. My big concerns in replacing the HDD (for a WD Black 3 TB in my case) were passing the Apple Hardware Test and more importantly what would happen to the iMac's internal fan speed. I had read that fitting a 3rd-party SSD would not cause the fan to go to max speed but I found little or mixed info regarding 3rd-party HDDs. An approved Apple service centre told me that there would be no fan problems fitting a new HDD. OWC on the other hand state that you NEED their propiatry SATA interface cable, complete with inline thermistor to stick to your new HDD or indeed SSD. I really didn't want to fit the OWC cable if not necessary and neither did I want the fan under 3rd-party software control. So here's what I found. I opened up the iMac and swapped the existing Seagate 1 TB for my new WD 3 TB HDD. Rebooted the iMac using an El Cap boot USB stick. Disk Utility repaired the Fusion Drive easily. The Apple Hardware Test failed, reporting a sensor fault. The internal fan went to maximum and stayed there. Boo. I switched off then plugged in the OWC lead, sticking the thermistor to the central boss of the HDD. Rebooted and.... success. The Hardware Test passes and the fans go back to normal - around 1200 rpm. The information that I've found (mostly from doesnotcompute.org) is that although Apple no longer use the extra 2-wire sensor attached to their iMac HDDs to report HDD temp to the system, they don't use the S.M.A.R.T system either. But there's no additional sensor and they only connect via a standard SATA plug - so what do they do? Evidently 2012 iMacs do use the HDD's in-built temp sensor BUT they pick it up from SATA pin 11, normally used to make an external LED blink. Apple got their suppliers (WD and Seagate) to write special firmware for their Apple-bound HDDs only to make this happen. Crafty buggers. The Seagate that I took out has 'Firmware - AP15' printed on it's label. So if you fit a 3rd-party HDD (even another Seagate or WD) the HDD temp info won't be going to pin 11 and the iMac won't read it = fan frenzy. I can't comment on what happens with SSD's, which to be fair seems to be what most people swap their spinners for - I would have put a 2TB SSD in myself, had they not been so crazy expensive still. Everything I've read suggests that putting in an SSD does not cause fan problems, although I dare say the Apple Hardware Test would still fail.