It's a long story. In August 2017 I replaced my 2011 MacBook Pro (mostly used in clamshell mode with a mouse, keyboard and external monitor) with a built-to-order 4k 21.5-in iMac. Because I'd specified 16 GB of RAM to go with the 1 TB Fusion drive it was classed as build-to-order as it wasn't on off-the-shelf unit. I used this regularly until I replaced it in late 2022 with an M1 Mac mini. In February of 2021 I had the fine folk at an Apple reseller and repairer replace the faulty 1 TB Fusion drive with a 1 TB SSD.
Meanwhile in November 2017 retired friends in their late 70s decided they'd had it with Windoze 10 and switched to an iMac. They bought the basic off-the-shelf 4k 21.5-in iMac with 8 GB RAM and a 1 TB 5400 rpm (slow) HDD. Over time and with the passing of her husband, my friend's compute became increasingly slow and hard to use. Then one of her Widoze "expert" friends decided he could fix a problem with her iMac but made it worse by creating a new user account and installing Norton 360 for Mac on it.
This proved so incredibly difficult to fully uninstall but eventually I was able to get rid of it. I then threatened physical harm to this Windoze "expert" if he ever touched her iMac again. It seems his meddling had made on old and slow Mac even older and slower. Just to boot it up meant finding something else to do for five minutes while it started up.
This friend with the sick iMac is the Men's Shed's honorary Treasurer. On Wednesday she presented the monthly report in handwritten form as the iMac had died partway through printing out the latest issue. It was now taking in excess of 12 minutes to boot and would randomly shut down, the only way to boot it up again was to disrupt the power supply before hitting the power button on the back again.
On Wednesday I told her I could offer the use of a similar iMac or a 13-in M1 MacBook Air, she rejected the latter as being too small. Yesterday I arrived at her place at 3 pm with my spare 4k iMac with me. I'd updated it to the latest version of Ventura and created a basic user account.
After determining that her iMac was beyond salvage I enquired about the location of her Time Machine external HD, eventually finding it hiding on a high shelf, it had last been used on November 2nd, 2023. By this stage her iMac had stared and was as stable as it was going to get. I managed to get a new Time Machine backup made. I then connected the external drive to my iMac and used Migration Assistant app to add all of the content of the two user accounts from he old iMac to mine.
This process took about 3.5 hours to transfer 498 GB of data from the two user accounts, leaving 438 GB of free space on the 1 TB SSD. This was wrong, it shouldn't have been using that much space. After much investigation I found. in the invisible main Library a folder of Screen Recordings totalling 492 GB, created by her late husband in early 2018! Once these were deleted and his Photos Library transferred to her user account there was now 936 GB of free space. This was declared a win. I then started Mail to finish importing some 68,000 emails from the backup drive While this was done she went out for a pizza for our dinner.
I'm happy to report that my friend has a working computer that boots up fully in under 30 seconds and doesn't dick about when doing stuff. I'll go back later today to make sure her printer is working and to hide the invisible files I had to reveal in order to access the Library in her late husband's user account. Once I was satisfied the restored computer was woring well, I deleted the basic user account. It still has two admin-capable accounts. In addition I was able to find a user for my old-but-still-good iMac which I have given to her as no-one else wanted it and it was too good to dispose of otherwise.