Hi Allan and thanks for the reply.
I tried with both the Intel driver, v2.5.3.34 (which gives me a bluescreen on first restart only) and what Dell provides, v. 2.5.0.19 as their latest. Same result with both.
I have some other USB3 hdd enclosures using the ASMedia G11-ASM1051 chip, and copying to these seems to work fine. So I'm thinking compatibility issue with that ARM MB86C301B chip. Anything more info I can provide?
I have a disconnection error on one USB3 port only, but that looks like a contact & cable issue, as it works better (external drive disappears seldom) with another type of USB3 cable and no issue at all on same port with USB2 cable and USB2 hdd enclosures.
