Hello Sylvia, greetings from Germany,
i have exactly the same problem. Yesterday i build up a new PC with the DH87MC-board. I ran into a few problems but mainly the system is running stable and fine. But one of my problems is the fact, that the board powers down in POST when my external USB3-HDDs (WD MyBook 2GB) are attached and starts over. I can plug them in after booting Windows 8.1. Then they are working properly. On the Motherboard is the 0047 Bios-version installed. I tried to plug them in seperately in the USB3 and USB2 Ports on the backside. I tried to plug them in over a USB3 Hub (passive) and a normal powered Hub. Makes no difference.
Here is the data you need:
Reference Number: 0226 0734
AA number and serial: AAG74242-402 BTMC33200MTQ
Processor model: Xeon E3-1230 V3
Memory: 2* Corsair 4GB XMS3 (CMX8GX3M2A2000C9) and 2* Corsair 2GB XMS3 (CMX4GX3M2A1600C9)
Power Supply: Be Quiet! Pure Power 430W L8-CM-430W
PCI-e device: EVGA Geforce GTX 780 SC ACX (P/N: 03G-P4-2784-KR)
External Devices: 3 External HDDs plugged in over SATA (with external adapters) and 3 External USB3-HDDs (WD My Book 1140 USB Device) 2GB each
OK, i hope, that you can help us quickly.
Many greets, Tom