I dug out my Disk Caddy (allows any sort of disc drive hardware to connect to a USB) and plugged the drive from the dodgy Humax into it. Initially there was no recognition of the drive being there and the drive felt to be vibrating a lot (by finger touch) but not at the 5000 odd rpm it should have been running at. After a while (about half an hour), as it warmed up, it was suddenly seen by my Linux Mint computer and three partitions or directories appeared. Two of only 4GB and 41GB which appear to be empty and one of 930GB which seems to be pretty full although I can't actually access any of the three directories in it. These are Music, Photos and Videos which it shows as "loading" but never gets there. The fact that I can read these tells me that the disc formatting is compatible with my Linux machine, which is a relief.
I have seen this before on a drive from a laptop computer. I accessed that one by warming the drive up to 50 or 60 degrees in the oven and then putting it into the caddy and repeatedly attempting to access it until, as it cooled down, it became fully readable for just long enough to copy all of the wanted data from it. Obviously a case of the oil in the bearings getting a bit claggy but softening up when warmed.
Right now I don't have a spare drive big enough to drop the 930GB of good stuff onto, the largest drive in my spares box is only 500GB. I think it is time to stop and to preserve what I have until I can source a 1TB drive and a couple of removable, internal drive bays for my big desktop machine on the basis that properly wired in drives might have faster data rates than an external USB caddy setup does. Then try the warm oven trick.
In the meantime I will pop a spare Sata drive into the Humax and see what it makes of it.
Thanks for the input from you folk that responded, you pointed me in generally the right direction enough to be worth an evening playing.
Rans6...................................................