RetireJapan

I try to DIY when I can, but after watching some videos, I think replacing hard drives with SSD in notebook (or iMac) computers is beyond me.

That said, I am using my 2010 27" iMac as a monitor for my 2018 Mac Mini. They sell the 27" for ¥10,000 at the recycle shop as well.

I'll jump in here to underscore the potential savings of running Linux (in my case, Ubuntu) to revitalize and old PC or Mac. I used to live and die Macs until they started making them hard to repair. My current PC is an older x260 thinkpad that I got used on Amazon (from Lenovo directly) for about 24000 yen. A lot of Thinkpads are far more modular and repairable, even if you don't know too much about them. In the context of this forum, it's a really cool thing to try to figure out.

I'm not sure how you fared with that Mac since your original post, but you might try 1) starting it in verbose mode (hold ⌘ + V upon startup) to see if you can get a more informative error message, or 2) try making a bootable Linux thumb-drive/USB and see if your Mac can boot to that.

The craziest thing I ever did was replace the fan on my T400. I literally bought a fan on Amazon, a screwdriver set from the combini and followed 4 drawings on the Lenovo site like I was building an IKEA shelf (this was before YouTube tutorials were a thing).

I recommend Thinkpads wholeheartedly.

I did some internal work on an older mac mini--replaced one of its two internal hard drives with an SSD, and upgraded the RAM to 16GB. Works fine, but it's maybe a 2009 machine, so no system updates for several years now. It runs fine, boots as fast as a new machine, but is hampered in that it won't run the newest mac software (the Photos app is where it really falls down for my needs).

It was originally the server version--two 500GB HDs. I replaced one of those with an equal size SSD.
(Make me an offer?)