using old computer as a hard drive
How do I use an older iMac as an external hard drive?
iMac Line (2012 and Later)
You can try Target Disk Mode: Transfer files between two Mac computers using target disk mode - Apple Support.
You will also need this cables/adapters:
- A FireWire 800 Cable: https://www.amazon.com/Pasow-FireWire-MacBook-Digital-Cameras/dp/B071KSLDJ8/ref=sr_1_2_sspa?dchild=1&keywords=firewire+800+cable&qid=1615003893&s=pc&sr=1-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExWlpBN0cxNjUxSU1GJmVuY3J5cHRlZElkPUEwMTQ1NDQ4Mjc5U1VHVkJNWllDMSZlbmNyeXB0ZWRBZElkPUEwNDIyMTY0M1Y2SUxCU09ZTTlLJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==. Make sure you choose the right one at purchase.
- A FireWire to Thunderbolt 1/2 Adapter: Apple Thunderbolt to FireWire Adapter - Apple
- A Thunderbolt 1/2 to Thunderbolt 3 Adapter: https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter.
And this may not work with the failed graphics. Be prepared to return.
Cheers,
Jack