Hard Drive failure iMac please help

Well the good news is you can fix your system!

Lets review what a Fusion Drive is as it often confuses people! It is two physical drives, a HDD which is the main drive and a small SSD which is strictly a cache drive. Both your systems ports HDD - SATA connection & SSD - NVMe/PCIe slot are used.

The HDD holds ALL of your data! The SSD is like a pony express way station offering a fresh horse for the rest of the run. Here the Cache Drive holds more frequently requested data blocks (that fresh Horse) but the alteration of these data blocks are written back the the HDD.

But lets look a bit deeper what are these blocks?? You just started your system so what’s being loaded the OS and if you have already enable auto load of anything that too will be grabbed from the drive. Depending on the size of the cache drive (Apple offered a few different sizes across the different models) once the drive is full thats what is held. Now most of this stuff is not getting altered, but if you don’t auto load then the application you use and its data is more likely to be loaded. Lets say its your 100+ page essay your history teacher is expecting! Well that full document will be held in the cache, but as you correct and polish the document the data blocks get written back, In addition Apple was smart always writing back every so often and lastly iCloud if you have it enabled!

OK, Bang! The screen goes wacky and as you discovered the HDD died!

So the more important drive of the pair went on you so sadly the data it held is gone! But you where good! You backed up often as you knew HDD’s wear out!

Now the tricky part replacing the HDD. Frankly its not overly hard to do! iMac 因特尔 27" Retina 5K 显示屏 硬盘更换 iFixit offers a kit with all of the needed stuff! Or you can replace your drive as you want maybe you want another HDD larger maybe or you want a different vendors drive than what Apple used. You’ll need the iMac Intel 27" (2012-2019) Adhesive Strips and the tool that comes with it BEFORE you start. Unless you have some cardboard to make your own wedge you might as well get this iMac Intel 27" Cardboard Service Wedge.

OK what about that sensor thing?? iMac Intel 21.5" and 27" (Late 2012-Early 2019) SSD Temperature Sensor You didn’t tell us which macOS you upgraded to. The reason why this would be helpful is Mojave and newer now use the newer S.M.A.R.T. method of temperature monitoring, you see your systems heat management system (SMC) requires access to the drives sensor and of course Apple contracted for a custom HDD so the internal sensor is exposed for their SMC services. 3rd party drives don’t offer this modification so SMC goes crazy! Slowing your systems CPU clocking to a crawl and ramping up the Fans RPM to protect your system, To be safe I would put it in.

Now the last install issue If you are replacing the drive for a 2.5” unit you’ll need an adapter frame.

But… What about that cache drive?? You’ll need to wipe it down if you want to create the Fusion Drive again. Remember this only works effectively with a HDD as its mate! Frankly if you want to push your system to the max I would replace the small cache drive with something bigger like 512 GB or 1 TB and making it the boot drive. Here’s the guide 27英寸 5K Retina 显示屏 iMac 固态硬盘SSD 更换

Consider just going with the blade drive pulling both Fusion Drive set drives!

Hi Dan, I have the same issue as the OP and had a followup question to your very informative post. Instead of upgrading the blade drive to replace the Fusion Drive set (HDD+Blade), is it possible to just install the HDD with an SSD and somehow disable the use of the blade drive cache? I'm sure an SSD boot drive will be more than enough for me, and it is much easier from what I can tell, to upgrade the HDD than it is the blade drive.