I think this is what I need. Could you give me more information on how to find SATA 6G SSD in a USB3 external enclosure, and how to configure it? Also, can I completely replace the internal disk with this external one, or will it need to continue working? It hasn't completely failed, but I'm afraid it will soon.,
Thanks,
Terry
Choosing the correct external SSD and enclosure will depend on what year model your iMac is?
see > Identify your iMac model - Apple Support
One option is the > https://eshop.macsales.com/shop/owc-mercury-elite-pro-mini/usb-c
Personally I have a couple of different SATA 6G SSD's in Sabrent USB 3.0 enclosures and they work as expected.
https://www.amazon.com/Sabrent-Tool-free-Enclosure-Optimized-EC-UASP/dp/B00OJ3UJ2S?
Here's one of many articles: https://www.macworld.com/article/671308/how-to-create-a-bootable-usb-macos-installer.html
But why such a huge drive? You need to have backups still and they still need to be a dedicated external drive. My external boot drive was 500GB and my HDD backups were 1TB each. I keep history otherwise, so my backup need not be all that much bigger than my system drive, internal or external.
It will not replace anything, it augments. That is if you leave the internal disk in place. You should and use it to store data.
With drives as big as you're considering, a natural tendency is to put everything on it and forget it. You still need a backup at least as big as the main system drive and that means two 6TB drives. Conventional wisdom says backups to be 2-3x the system drive in size. That's 12-18TB. That is going to get a mite expensive if you go that route
Replacing Internal Disk with external SSD
