A couple of questions now that I see what you are doing and maybe a couple of things for you to try.
Questions first. Do you have multiple drives and is RAID enabled? When "Cutting" the backups (if you did), were these cut to the USB Drive? Did you try this, but first disable USB 29 (See Page 5-10) of the 8790 Manual and enable the Legacy USB Support in the BIOS Advanced Menu? When you disable USB Device 29, the only USB port (either external or internal) enabled wil be the one on the Lower Rear of the 8790. You will need to use that port.
I as going to try with my external stups; however, am in the middle of creating some VCR's to DVD's for a client and they are due tonight.
Now a couple of things to try. First, let's get Drive Image out of the picture. See if you can get a CD ISO of MemTest86, burn that image to a CD (Using the original machines CD-R burner) and then try to boot off it after disabling the other USB ports.
Forgot to mention the first device in the "list" of bootable devices should be the "External Devices".
I did a very brief "snoop" and found the following: http://www.dealtime.com/xPR-Powerque...on_PC_DM60ENK1
Indicates that this version of the software does not support USB. Look at supersnapps comments as they appear to be the most relevant - read all is suggested.