You are not alone GreggMack.
I have had this exact issue myself on a M6E board.
Three different flash drives, a Transcend 4GB drive, an old Lexar 1GB Jumpdrive and a newer Corsair 16GB Voyager.
I have had the system crash when plugging in any of those into the front or rear USB 2.0 ports.
Similar to Gregg, I'd say it happens around 1 time out of 4 for me. (25%)
I do not have my system built out yet.
I just found a stable OC and only have Windows 7 Enterprise and basic drivers installed.
The only software on it is CPU-Z, HWMonitor and RealBench.
That's it.
I have also loaded Windows 3 different times in the past three weeks.
It has done it with every install.
Board is on UEFI 1102
One interesting note, I have never had it crash if I put in the flash drive and then power on.
It only crashes if I plug them in while in Windows.
Main Rig
i7 4770k OC 4.4GHZ 1.2875V
AirCooled with Noctua NH-D14
Corsair Vengeance Pro 2400Mhz 16GB (2X8GB)
Asus Maximus VI Extreme
Asus GTX 780 Ti
1 Samsung 840 EVO 500 GB SSD+ 3x3TB WD Reds in RAID 5
Seasonic Platinum 1000
Corsair 750D
Dual Asus PA248Q monitors
Windows 7 x64