I finally found the time to have another play and find the following:
I used Clonezilla to create a copy of the 500GB drive from the good machine. I tried it in the working machine and it made it work normally. Good.
I put the copied drive into the 1TB faulty machine and it "started" but never completed booting up, resetting and re-starting at about 5 min intervals.
I then put the copied drive into the good machine which booted up and then behaved like a new machine by updating the OS and re-tuning all of the programme channels. It then worked normally. All of our recorded programmes were apparently preserved intact, I only watched a few minutes of one to check but everything seemd OK.
Some drives are not 100% compatible with the disk caddy! Something I suspected from before.
Clearly the faulty 1TB machine does something to a good 500GB drive when I is installed which makes it obvious to the good 500GB machine when installed there, I don't know what.
The 1TB machine does not initialise a blank 500GB drive, again I don't know if it would do to with a 1TB drive but I am going to find out soon.
I am not in a position to see if the good 500GB machine does anything useful with a blank 500GB drive.
A close inspection of the innards of both machines shows that both have the same iteration of the same circuit board. The circuit boards have no obvious option links or switches but they may have information programmed in which stops them from working with the wrong size HDD.
The disk partitions are similar between the 500GB and 1TB drives, the two smaller ones are the same sizes, only the large video storage space is changed.
I have got no further with warming the faulty 1TB drive to see if the data can be cloned. Until the 1TB SSD arrives I have nowhere big enough to clone it to.
I have a work around which takes the disk caddy out of the cloning process.
Rans6.....