From there, the disks can be accessed as simple attached devices. A non-mac operating system will ignore the disk entirely and boot the machine just fine. Long story short, deep level disk corruption can apparently sometimes interfere with the normal mode of operations that macOS uses to boot from recovery media. Then Internet Recovery Mode worked fine, and I re-partitioned the disk and restored from backup no problem. However, I was able to boot up using a USB Ubuntu live distro, which worked fine. I don't fully understand the nature of the OSX recovery boot scheme, but attempts to boot to Internet Recovery Mode or to an OSX USB boot stick try to query a part of the original disk that was corrupted and induced a kernel panic. They were either incompetent, scammy, or both.

Not to be snarky here, but this was an 'Apple Authorized Service Provider!!!' whom I scheduled using the Apple website. I'm wondering what my options are: new Mac, replace the hard drive or get an external one - would that be about it? If the hard drive were to fail would it be better to have it replaced rather than use an external drive (that I could move between Macs)? I just ran an Etrecheck report on it & it says the hard drive appears to be failing, which says I have a more serious problem than it just being annoyingly slow. He said it might also be possible to replace the internal drive with an SSD depending on what 2017 model it is. Is it more complicated than I think to run the Mac off an external drive? It doesn't seem that complicated to me to download the OS onto another drive & make that the startup drive, so I am not sure what he meant. I've been thinking that maybe I could get an external SSD and use that as the startup drive instead of the inernal drive but the guy in the local mac shop told me this would be difficult to do unless I knew what I was doing and expensive ($850Aus).

it has always been slow and I've put it down to not getting an SSD when I bought it.

'Hard drive appears to be failing' - what are my options I have had this iMac for just over 3 years (2017 21 inch, 4K).

Cost to replace hard drive in imac