Thank you very much.
Indeed the connected drivers are very volumineous - 3TB and 3.5.TB.
I think the USB connectivity is to be exluded. They work(ed) well with the USb attached drivers before and at present there is no problem with connection of a small-data-contains USB pendrive or an (almost) empty WD My Password TB.
I have just spoken to a WD technician. As usual the (almost) only and standars proposition is to replace the disk.
They do no intervention in SHH (as you can see I cannot even spell it correctly). They have (I have spoken to the first level support staff) no serious idea of the origin/source of the problem.
I have eplained the issue connection of a large volume USB external hdd and the potential problem of indexing - no reaction, or advice on the very issue. It is a shame. I suppose I can get more information from this forum that by direct conversation with WD Technical Support staff. I am very dissapointed.
I have been waiting for the presentation of the problem to a higher lever and its results.
As far as SSH is concerned I am affraid of loosing a warranty of this unfortune product ( putting it mildly) and above all I am not a Linux expert at all. In this case I would need a step-by-step clear and a very correct instruction of making some changes there.
Once more thank you very much for your present pieces of advice and (if any) those consequent ones.