I thought I'd throw out some of my personal experiences in the last 12 months with Intel iMacs, 2.5" SATA SSD drives and Fusion drive data recoveries.
As a backgrounder, Apple never fitted a 2.5" SSD's to these iMacs. Apple's SSD option was a system of 32GB NVMe blade drive (NGFF connector) and Seagate 1TB 2.5" HDD, married together in software to become a Fusion drive. Non Fusion systems have just the 1TB Seagate HDD.
So, the most common reason an iMac comes across my bench is because it's either ridiculously slow, or it fails to boot at all. The most obvious reason is failure of the Seagate 1TB HDD with bad sectors, and most will just replace this with a 2.5" SATA SSD and be done with it. That is up until recently I've been having a lot come in (or bounce) where a SSD has already been installed. When tested with Blackmagic's Disk Speed Test, the performance figures will be down to almost nothing.
Replacing the drive will solve the issue in the short term, but as I have seen, the drive will likely fail again. Either within a few weeks or up to 6 months. This failure is seen much faster on SSD's with NAND based cache (such as Crucial BX500), but I've had a number of DRAM based cache (such as Samsung 870 EVO/QVO) come in that are also very slow or fail to boot after just 3 years of use.
Something about MAC OS is killing these drives, as I've not seen this issue in Windows systems. This is despite the TB written values being well under the spec. For example, I have a Samsung 870 QVO 1TB SSD with just 12TB written that is running super slow and manufactured in 2021. Spec for that drive is 2,880 TBW! So this is well short. I don't know what is causing the problem from MAC OS, but I have am going to try disabling Spotlight search from here on to see if this makes a difference; possible the drive indexing is causing some issue not accounted for in the firmware. I will touch base with Crucial and Samsung for comment.
BTW, I have had the same issues whether SSD Trim was enabled or not. Trim actually seems to make it worse to be honest.
iMacs from 2013~15 usually have the NGFF connector for the Blade drive fitted on the motherboard. In this case, I remove the board and fit a Crucial P3 Plus M2 drive with NGFF to M2 adaptor and don't bother with the SATA. Haven't seen these come back (yet). Time will tell, but transfer performance is so much better due to the MAC using 2 or 4 PCIe lanes for transfer. 2017 iMacs usually don't have the NGFF connector populated if it wasn't ordered with the Fusion option. In this case, I strongly recommend you only use DRAM cache drives such as the Samsung 870 EVO. I wouldn't recommend the QVO currently as this is a QLC device. NAND based drives, such as Crucial BX500 and the cheaper rubbish from Taiwan (Silicon Power, Patriot etc) should never be used.
Now from a data recovery aspect, the Fusion setup is a complete nightmare and a total PITA. The scheme used by Apple is proprietary and AFAIK, there is no documentation from Apple or diagnostic firmware for the blade drive for data recovery. Most common thing I see is bad sectors on the HDD. As the HDD is part of the fused file system (the bulk of which is on the Apple blade drive) you are limited to what you can do outside of the iMac. Put the HDD in a PC and R-Studio doesn't see a file system for example, just raw data.
What I have done successfully in the past, is use HDD regenerator to recover bad sectors (one drive took over a week for that to finish), dump the raw HDD data to a file (using the HDD Raw Copy Tool), copy to another working Seagate 1TB and reinstall to the iMac. I prefer using a file then copy, as this minimizes I/O time and wear on the already degraded HDD. Usually the system will boot up ok, or I launch it in Target disk mode and dump the files off manually to another MAC. This works for Fusion and non-Fusion systems. On no account modify the Apple blade drive, as this has the file system on it.
The most recent one, a 2017 A1419 iMac, had a Samsung 870 QVO 1TB SATA drive fitted (dated 2021). The last repairer thought he was doing the right thing and merged this with the Apple 32gb SSD as a Fusion system. Now the system would crash during loading MAC OS and boot loop. I could mount the iMac in Target disk mode and started dumping off files. But this would hang at some point in the transfer. Transfer this to my workshop PC, and the Samsung drive seems perfectly fine in the Magician software with only 12TB written. However, when I went to dump the raw data off, the drive was pretty slow. At times it would seem to hang, then start up again. I'm not sure if this is some kind of error correction on the drive or the software. It ultimately dumped all the data off and I copied this to a new Samsung 870 EVO (same exact size) drive. I reinstalled the replacement drive and was able to dump off the users data via Target Disk mode.
I do not recommend Fusion setups at all for iMacs now. Highly recommend to any customer that comes in with them to get rid of that setup as fast as possible.
As a backgrounder, Apple never fitted a 2.5" SSD's to these iMacs. Apple's SSD option was a system of 32GB NVMe blade drive (NGFF connector) and Seagate 1TB 2.5" HDD, married together in software to become a Fusion drive. Non Fusion systems have just the 1TB Seagate HDD.
So, the most common reason an iMac comes across my bench is because it's either ridiculously slow, or it fails to boot at all. The most obvious reason is failure of the Seagate 1TB HDD with bad sectors, and most will just replace this with a 2.5" SATA SSD and be done with it. That is up until recently I've been having a lot come in (or bounce) where a SSD has already been installed. When tested with Blackmagic's Disk Speed Test, the performance figures will be down to almost nothing.
Replacing the drive will solve the issue in the short term, but as I have seen, the drive will likely fail again. Either within a few weeks or up to 6 months. This failure is seen much faster on SSD's with NAND based cache (such as Crucial BX500), but I've had a number of DRAM based cache (such as Samsung 870 EVO/QVO) come in that are also very slow or fail to boot after just 3 years of use.
Something about MAC OS is killing these drives, as I've not seen this issue in Windows systems. This is despite the TB written values being well under the spec. For example, I have a Samsung 870 QVO 1TB SSD with just 12TB written that is running super slow and manufactured in 2021. Spec for that drive is 2,880 TBW! So this is well short. I don't know what is causing the problem from MAC OS, but I have am going to try disabling Spotlight search from here on to see if this makes a difference; possible the drive indexing is causing some issue not accounted for in the firmware. I will touch base with Crucial and Samsung for comment.
BTW, I have had the same issues whether SSD Trim was enabled or not. Trim actually seems to make it worse to be honest.
iMacs from 2013~15 usually have the NGFF connector for the Blade drive fitted on the motherboard. In this case, I remove the board and fit a Crucial P3 Plus M2 drive with NGFF to M2 adaptor and don't bother with the SATA. Haven't seen these come back (yet). Time will tell, but transfer performance is so much better due to the MAC using 2 or 4 PCIe lanes for transfer. 2017 iMacs usually don't have the NGFF connector populated if it wasn't ordered with the Fusion option. In this case, I strongly recommend you only use DRAM cache drives such as the Samsung 870 EVO. I wouldn't recommend the QVO currently as this is a QLC device. NAND based drives, such as Crucial BX500 and the cheaper rubbish from Taiwan (Silicon Power, Patriot etc) should never be used.
Now from a data recovery aspect, the Fusion setup is a complete nightmare and a total PITA. The scheme used by Apple is proprietary and AFAIK, there is no documentation from Apple or diagnostic firmware for the blade drive for data recovery. Most common thing I see is bad sectors on the HDD. As the HDD is part of the fused file system (the bulk of which is on the Apple blade drive) you are limited to what you can do outside of the iMac. Put the HDD in a PC and R-Studio doesn't see a file system for example, just raw data.
What I have done successfully in the past, is use HDD regenerator to recover bad sectors (one drive took over a week for that to finish), dump the raw HDD data to a file (using the HDD Raw Copy Tool), copy to another working Seagate 1TB and reinstall to the iMac. I prefer using a file then copy, as this minimizes I/O time and wear on the already degraded HDD. Usually the system will boot up ok, or I launch it in Target disk mode and dump the files off manually to another MAC. This works for Fusion and non-Fusion systems. On no account modify the Apple blade drive, as this has the file system on it.
The most recent one, a 2017 A1419 iMac, had a Samsung 870 QVO 1TB SATA drive fitted (dated 2021). The last repairer thought he was doing the right thing and merged this with the Apple 32gb SSD as a Fusion system. Now the system would crash during loading MAC OS and boot loop. I could mount the iMac in Target disk mode and started dumping off files. But this would hang at some point in the transfer. Transfer this to my workshop PC, and the Samsung drive seems perfectly fine in the Magician software with only 12TB written. However, when I went to dump the raw data off, the drive was pretty slow. At times it would seem to hang, then start up again. I'm not sure if this is some kind of error correction on the drive or the software. It ultimately dumped all the data off and I copied this to a new Samsung 870 EVO (same exact size) drive. I reinstalled the replacement drive and was able to dump off the users data via Target Disk mode.
I do not recommend Fusion setups at all for iMacs now. Highly recommend to any customer that comes in with them to get rid of that setup as fast as possible.