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Backstory: The 3TB HDD failed in my ‘Late 2012, EM2546, 27" iMac’ and i need to replace it. I got my machine asssessed from the Mac Center (an authorised repair center here in Sydney, Australia) and the quote for repair only mentioned replacement of the 3TB HDD and nothing about the PCIe SDD, so i am safely assuming its only my HDD that needs to be replaced. But, as a side note i am also looking into (but undecided on) upgrading the PCIe Blade SDD. I’ve done a fair bit of digging around online and on the iFixit website and understand the processes involved in physically replacing the main HDD, and the PCIe Blade, alongside supplementary pieces of hardware needed (OWC Thermal Sensor etc.). …My queries are more geared towards the steps thereon after physically replacing the HDD (and potentially the PCIe Blade)… So finally, my Question is: After replacing the main Hard drive and/or PCIe Blade, is it as easy as turning on the machine and starting with the same menues and setup procedures as you would be doing if you were starting your iMac for the first time? and just going through the motions? Or Is there more to it and are there additional things that i need to do for me to have the end goal of having a running machine with a new HDD (and/or potentially a new PCIe Blade)? I understand this a giant query and do extremely appreciate the help that anyone and everyone can give me. Thank you for your time guys!

Here’s the list of storage upgrades available for your machine: https://eshop.macsales.com/upgrades/imac…

To start with here’s a bit more on the Apple blade SSD’s The Ultimate Guide to Apple’s Proprietary SSDs Cody need to add to it but its not bad! Focusing on your question what happens after you’ve swap out your drives. Well if you have a blade SSD presently then you have a Fusion Drive setup. So leaving it may create some issues with the new HDD as the SSD drive will boot up your system and assume you want to have a Fusion Drive setup again. But not all of the OS & App files will be on the SSD so you may encounter issues until you wipe the drive down and re-install the full OS and Apps. The next issue is Apple does not support Fusion Drive with High Sierra so you’ll need to stick with Sierra until Apple addresses this (hopefully in the next update). OK, lets say you pull the old blade SSD and since your HD is dead we have a virgin system then with new drives. Hopefully you have access to a working Mac so you can create a OS installer USB drive. Here’s the Sierra guide to create the installer drive: How to create a bootable macOS Sierra installer drive. With the installer in hand you can prep the drives formatting them for GUID partition mapping and Extended (Journaled) file system partition. Lets say you don’t have access to a second Mac what then? Well if you have access to the internet (fast connection being best) you can jump into internet recovery (see below) this will allow you to install the OS from Apples servers. As you don’t have a configured HD the hidden recovery partition won’t be present so thats not an option. I’ll wan you this can take some time which is why I really recommend setting up the USB OS installer drive. Mac startup key combinations Then the next question is which way to go with a dual drive’d system back to a Fusion Drive config or keeping them independent? The size of the blade SSD is the key here. A 256 GB SSD or smaller makes sense to setup as a Fusion Drive. If you have a larger SSD then it makes sense to keep them independent. Here you setup the SSD as your boot drive and then copy over your user folder to the HDD. Here we want to then create alias folders pointing to the HDD folders. The folders you can move are the Documents, Downloads, Movies, Music, & Pictures. You want to leave Applications, Desktop & Public really should stay on the SSD for the best performance. For reference my iMac has a 512 GB SSD and a 1 TB HDD setup a discreet drives. After you’ve done the OS install you’ll be asked if you have any user accounts, apps & user data to migrate over this is the time to migrate the user accounts and Apps’s. Once you’ve moved the folders to the fresh HDD then go back in to the Migration Assistant to then copy over your users data files over from your backup drive.