The external usb enclosures both have their own power.( Have had same problem with two different enclosures, different makes). Second drive just started failing but when connected internally via SATA problem disappeared. I have been reading on net that people have this problem with external enclosures and I just don't want to mess with it.
This molex to SATA seems to be what was missing. The dell PSU cable still has a four pin connector empty.
So I'll plug another drive into second motherboard SATA port and power it with a molex to SATA cable. That gives me a system drive and two internals that I can swap as they fill up. I'll only use USB for occasional use.
I'll just set the third drive on top of the cabinet, securely, there are no more bays.
That solves my problem if this scenario seems plausible.