Fitting an old 1tb drive inside the Mac was relatively straight forward. However, I discovered that the Mac could not see the drive as it was limited to 150MB/sec speed drives and the Western Digital Green drive I used did not auto-fallback to this speed.
The drive has that rate disabled by default, therefore it can’t be seen unless you enable the 150 MB/sec rate on the disk. One does this by jumpering pins 5-6. Unfortunately, the drive did not come with any jumpers and finding one that fitted correctly proved difficult to source.
I gave up on this and tried a 1tb Hitachi drive which auto set the correct speed. The drive was identified in Disk Utility and after a quick format I was ready to install the OS.