I have two 1TB external HDDs, a WD and Seagate one.
When I connect them to the USB, motherboard GB EP45-DS4, the system reboots during POST. This happens when the ext HDDs start to spin/spool up during the memory test message. Once I remove the ext HDD connections, it worked fine.
You found the problem!
It is a Gigabyte motherboard problem.
No solution at this point.
Yup. Gigabyte boards have a tendency to have this problem. I think what's happening is that the BIOS is trying to boot from the USB drive and since it can't find the boot files it reboots and we all know when rebooted multiple times in a short period of time it clears the CMOS. However, if I have a Linux Live USB things work fine (as in it boots to Linux with out problem,etc). Hence, my conclusion.
Like you it's happened to me a few times on multiple Gigabyte boards using the P35/P45/X38 chipsets.