|"a different hd works fine, so the cable/usb port/housing device/power can be disregarded as probable causes."|
You MUST be sure the USB port you have it plugged into can supply enough current, however, if another external drive works fine, then the USB port probably can - it has to be able to supply the full USB spec 500ma - e.g. a port built into the mboard.
The power adapter for an external 3.5" drive MUST be plugged into the enclosure and be working properly. Some 2.5" external drives will NOT work properly unless you make two USB connections on the computer end, or one and also plug in a power adapter to the case.
The external enclosure's circuits may be damaged.
You MAY need to buy a new external enclosure, or connect the drive internally to a computer, before you can attempt to repair the data damage.
If you have another drive you can install in it so you can test whether the enclosure works fine, try that.
If you can't do that and have any doubts about that, connect the drive internally to a computer before you attempt to repair any data damage. According to my experiences you can't run the hard drive manufacturer's diagnostics on the hard drive when it's in the USB enclosure.
Testdisk - get the Stable version:
PC Inspector (freeware)
Zero assumption Recovery
The demo is limited
It will only recover 'up to' four folders per run
But you can make multiple runs
Really good, but you have to pay for it.