Hah! Had the *exact* same issue myself a year ago. Isis XL.
Ended up taking it to a local repair shop who opened it up and told me they could find nothing wrong (and that'll be $80, please)
And it too worked when I USB plugged it in when I got it back, as "one last try" before sending it to x-rite for a $750 quoted repair.
It's very odd that it needs to be USB connected *and* power lead, to get the green power light. Nothing else I have behaves this way, but... it is X-Rite we're talking about here!
ps: Mine was making clunky stepping noises as it read the patches, while still under warranty, so I got a free service-fix out of x-rite, at least.