This is what happened:

My first M4E-Z got killed by water (cooling) after a couple of years with no real problems.
I had a spare board, and set that up air cooled - and it worked ok for a month. Then it was waterblocked - immediately after got about 3 BSODs - all FE USB_BUGCHECKs, and all had a first parameter of 2. This means that:
The USB client driver has submitted a URB that is still attached to another IRP pending inthe bus driver.
I could not drill further down than it was caused by hcmon. The IRP that caused the bugcheck was not known by Windbg (if anyone knows how to offset, or otherwise get the device associated with the IRP then that would be great:-))

Ok, that looks like it might not be a hardware prob. I thought. And the system ran for 1.5 days before the same BSOD struck again. The next day (today in real time) got 3 consecutives.
So I'm thinking the only way it can not be hardware is if I installled a driver/virus etc within a couple of days before installing the waterblocks? Bah.

Since the 5-Extreme will work with the same CPU/RAM as the 4, I’m wondering if it might not be worth getting regardless – that is, is the 5E generally better than the 4E-Z? For example the firmware for the USB3 devices cannot be updated in 4. I’m guessing they can in 5.

But if anyone has a fix for the USB BSOD, or can say it cannot be hardware, given the 1st parameter, I will be very happy.
The thought of reinstalling everything – check ok, then waterblocking fills me with dread.