I knew I could copy files off it, so thought about whether I should start that. However that would mean using multiple drives and using another Mac. I knew that this would take time, as moving files from one mobile drive (well the iMac in disk mode) to another takes much longer than moving them from the internal HDD to an external HDD. The Mac mini I was using to connect to the iMac only had a small SDD so wasn’t an option.
In the end I decided that I would buy a new external hard drive.
I went with the LaCie STHG4000400 External Hard Disk Drive. I went with the 4TB version in silver.
Though I went with the USB-C interface, I knew that included was a UBS-C to USB 3.0 cable I could use with the iMac.
Having ordered that with next day delivery I turned the iMac off and left it alone.