At the moment I think the enclosure is set up/connected USB 2.
Sorry - but I will be out of town and away from the system for at least a couple weeks: I can remote in via my PD connected to the server, so will at least hope to get all the current drives "checked" over that period of time, then be in a position to restart server, check BIOS setting to be sure I really have USB3 enabled, and start again from there.
This all started (i.e.everybody was playing nice) until about 6 weeks ago when I had an old (>3-4 years) 1.5 TB drive go bad, overheat, and die. I've replaced it and actually managed to migrate all the data from the remaining 3 drives in that enclosure, however have had recurring problems since.
And yes, some of my data is photos (jpeg and a few mpg) but the bulk of data stored is videos (iso) files that are big and space-hungry.
Oh, if only HP still made a MediaSmart server which was soooo reliable - but it's motherboard dies (after 7-8 years) and so I built my own server. Woe is me.
I'll reactivate this message chain when I can get back hands on.
In the mean-time thanks for all your suggestions and directions!