It's not the enclosures
Enclosures A & B are the same model. They support 3tb. The 3tb drive will work in whichever enclosure it is formatted in. It loses the format when it is switched between A & B. So, the problem is not whether the enclosure supports 3tb.
Also, A & B have no problem switching back and forth drives that are 2tb and keeping the 2tb formatted. So, that would indicate an interface chip is not the problem.
Also, the reason I buy Internal HDs and put them in enclosures, is to afford me the luxury of switching enlosures when the enclosure breaks. Not being able to remove the internal HD from the enclosure defeats the purpose.
I think I'm just gonna RMA the 2 Toshiba 3TB drives.