New observation!
I don't know if this apparatus: “Sabrent USB-DSC8" has been making good contact through any of this process. From the beginning I was having some difficulty plugging the SATA conector into the Sabrent adaptor, and finally noticed that the internal part on the adaptor is not straight, or even. When you plug it in one side moves away so that those connections may not have been making good contact (connections 1-3 or so, see:
http://www.interfacebus.com/SATA_Pinout.html).
I tried a different Sata drive, with the same inadequate results, and then (using the same Sabrent adapter, but the ATA part of the connector, and an ATA (or IDE) drive, and made a good connection and was able to open, and even format the ATA drive.
I tried an older adapter that I had here, with the 3TB drive and got a completely different reading. Here is the result of the command: dmesg | tail after plugging this older one in with the 3TB drive:
$ dmesg | tail
[ 520.263294] sd 11:0:0:0: [sdd] Write Protect is off
[ 520.263302] sd 11:0:0:0: [sdd] Mode Sense: 00 38 00 00
[ 520.264277] sd 11:0:0:0: [sdd] Asking for cache data failed
[ 520.264285] sd 11:0:0:0: [sdd] Assuming drive cache: write through
[ 520.267567] sd 11:0:0:0: [sdd] Asking for cache data failed
[ 520.267583] sd 11:0:0:0: [sdd] Assuming drive cache: write through
[ 520.703094] sdd: unknown partition table
[ 520.706213] sd 11:0:0:0: [sdd] Asking for cache data failed
[ 520.706227] sd 11:0:0:0: [sdd] Assuming drive cache: write through
[ 520.706238] sd 11:0:0:0: [sdd] Attached SCSI disk
clearly, still a problem, but a different one it seems. ("sdd: unknown partition table" doesn't make much sense, since this drive works fine as an internal drive, on the same machine and OS).
I'm going to send this thing back, and put this problem 'on the shelf' for now, until I have a better waysdd: unknown partition table to pursue it - unless this bit of news sparks a comment that will set me straight on this.
Thanks again for all your suggestions. And sorry that I didn't catch this sooner.