You would need the "active" kind of riser, that assigns PCI addresses to the extra slot(s). Otherwise, I'd think it should work. There might be space in the front chassis extension to remount the HD there to gain some headroom.
Each PCI slot has some unique address lines so that the system knows what card is plugged in what slot. Most multi-slot risers will plug into one slot, but have cables that plug into adjacent slots to pick up the address lines. I.e. 3-slot riser needs 3 empty slots on the M/B. There are cards (I can't find a link to one right now) that have circuitry to "synthesize" the extra addresses so you only need 1 available slot on the M/B. That's the kind of card we would need.
No, just that the expansion boards couldn't draw more than 2A total of 5V (10 watts total) I don't know if that would be a big limitation, but it sounds like the power distribution circuitry of the motherboard isn't designed for a lot of expansion...
An FYI related to existing riser. The tuner/video card that I used is touching the HD I used and separted by a thin sheet of plastic. Best to use a smaller footprint (ht) HD or even a notebook drive. Cutting slot out for video card wasn't that easy with dremel. Much more difficult than the slim CDRom cut out as the metal is slightly thicker.
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