I am still undecided over this issue. The Humax machine did not attempt to do anything to a brand new, unformatted HDD as far as I can determine. This leaves me with two options:
1) to purchase a 1TB drive and attempt to do a clone copy. This assumes I can persuade the drive to operate by warming it a bit. I already know that it starts to work, if left connected and you keep trying to access it, as it warms up.
2) to purchase a 1/2TB drive and clone copy the drive from our other Humax machine of the same model.
In either case I am leaning towards a SSD rather than a HDD as the machine accesses the drive all of the time it is powered on and it makes a slightly annoying chortling noise. An SSD should be silent in use.
I still don't know if it is a quirk of the disk caddy that stops my Linux mint machine from seeing a totally blank drive. Maybe if mounted internally it will be seen?
I have obtained Clonezilla cloning software to allow the two options to be tried.
Rans6......