iMac 2017 rebuild options

To fix the split Fusion drive and reinstall the macOS.

see > How to fix a split Fusion Drive - Apple Support

Then migrate from your backup during the initial macOS setup.

see > Transfer to a new Mac with Migration Assistant - Apple Support

first, be sure you have your USER and any partitions backed up on reliable separate drive(s)?

i would have two backups -- a TimeMachine and cloned copies (because you're going to reformat your current working drive)

next -- what macOS is currently installed (Mojave...Ventura)?

what macOS do you want to install on your 2017 iMac?

here from another post:

download the preferred macOS installer

How to download and install macOS - Apple Support

use Terminal to create a bootable macOS thumbdrive installer

(scroll down midway on above link)

i would recommend once the installer boots:

open Disk Utility then Erase the internal drive (where current macOS is installed)

format APFS for SSD (actually the installer may default that option when you run the installer)

then run First Aid on the erased empty drive

then run the installer...

see den.thed>> Then migrate from your backup during the initial macOS setup.

see > Transfer to a new Mac with Migration Assistant - Apple Support

--hit back if you don't understand something--

I WOULD AGAIN HIGHLY RECOMMEND upgrading any HDD (spinning hard drive) to a SATA SSD -- macsales-com OWC has great instructional videos -- i would suggest SATA 2.5" Samsung 870 EVO and minimum 16GB RAM (ram also available on macsales-com)

i usually preserve the original HDD, install the new SSD, then use Migration Assistant to bring everything over from the original (via an external USB enclosure)

I had a 2017 27" iMac with an SSD drive. However, I had a couple of external SSDs which I had cloned my earlier systems on, High Sierra and Mojave. If you can get the Fusion drive fixed you can clone it to an external SSD (an MVme SSD would be the fastest: Using Carbon Copy Cloner to Make a Bootable Clone of Intel Mac.

Be sure to get an external SSD that's the same size as your current Fusion drive or larger. You will experience speeds that are at least 4-5 faster (probably more) than what you've got now. I recommend getting it from OWC (MacSales.com). They are considered to be the premier hardware supplier for Macs by most of the experienced users here.

You’re welcome.

I have the same computer. After it started slowing down so that it was practically unusable, I set it up to boot from an external drive running the operating system. My concern was that the operating system was getting old and stopped upgrading.

Eventually I got a new computer and set the iMac aside. As an experiment, when I got ready to junk the old one, I carefully pried the screen off to see if I could remove the failing hard drive and was surprised to be able to replace the Fusion Drive with an SSD. This speeded it up and made it fast and usable again. I taped the screen at the top to the computer body. It isn't pretty but it works. There were two ribbon cables attached to the motherboard that had to be pried off, then reconnected. Again, I had a new computer and nothing to lose by taking the old iMac apart. I only use it with a webcam to capture night recordings and don't care about a new operating system.

I wouldn't have attempted this if it was my main computer - but it can be done. I am just offering options.

* I tried the same thing with a much older 27" iMac and ended up cracking the screen.

You should be able to run the 2017 iMac from an external SSD while you consider your options.

Use an external SSD as your startup disk … - Apple Community

The 2017 iMac is obsolete and I would replace it with a new iMac or Mac mini.

Mac - Which Mac is best for me? - Apple

simply

just Erase the SSD/HDD and reinstall the macOS

use Migration Assistant to bring your user over from your TimeMachine or clone backup

BEFORE erasing the current drive -- be sure you have your stuff in some form of reliable backup...

if you need a actual "repair" person for a "rebuild" option -- have them cut the screen off, blow out the dust, swap the SATA HDD with a SATA SSD -- that's good investment if you plan to keep using the machine...

ps

i used to be a fan of partitioning drives -- not any more -- your 2017 iMac supports THUNDERBOLT 3 -- look into an ACASIS TBU405 Pro enclosure and WD_BLACK SN850X NVMe SSD

that combination is 35 times faster read/write than the original HDD that shipped with 2017 machines

it could also serve as your boot drive...

Are you able to boot into the Recovery volume and do you have a Time Machine backup of your iMac? If yes to both boot into the Recovery volume and select Disk Utility from the options. Then run Disk First Aid on the fusion drive. Once you get the OK erase the fusion drive as follows:

Then install the system again. Then you can connect the Time Machine to the iMac and boot up. When it comes time to migrate you data do so.

Note: have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac? If so uninstall them according to the developer's instructions. They are not needed and can cause problems.

If you decide to forgo the repairing of your iMac and get a new Mac consider the following: a 10 Core Mac Mini M4 with 16 GB RAM and 1 TB SSD ($1084) with a 32" LG 4k monitor ($331) from Amazon.com)  is $1415 which is $541 less than a similarly configured new 24" iMac ($1956)  re 07/2025. Keep your keyboard and mouse from your current Mac for use with the Mini. 

I got the above Mini with a 32" LG 4K monitor w/speakers for only $60 more than the 27" (didn't know the Mini had a speaker).  There are many monitors available in 27" size on up for $100 to $500 depending on what features you want.  The monitor prices are before tariffs.

If you have a bootable clone backup made with Carbon Copy Cloner or SuperDuper!, you could try starting up from it.

Just a clarification, Apple considers a 2017 iMac "Vintage". Depending on which model of 21.5" 2017 iMac, Apple won't classify it as "Obsolete" until 2029 or 2031.