For drive bays there is a mounting panel on the left and right, the left according to the documentation is for 2 drives and on the right for one (all 3.5" mountings, no 2.5" laptop drive mountings are provided) and a central space for the optical drive.
It's possible to just make out some foam padding near the PSU, that's a sheet of noise dampening material I had left over from another build and I've covered all the hard to access areas for now while I am building and we'll see if more is needed on completion.
As you can see adding wiring start to gets a bit tangled quickly. One review I read suggested that a modular power supply is required, I think that is a bit OTT but would certainly make life easier. I've also found that the 20+4 pin connector a bit stiff for connecting to my mainboard and along with the extension lead for attaching the IR was dangerously close to being just too much wire blocking access to the connectors on the back of the optical drive, thankfully SATA connectors are so much smaller than IDE that with right angled connectors it all fits (incidentally 2 SATA cables were provided with the board which has 4 6b/s SATA connectors mounted at right angles to the board, which further points cables at where drives will sit, that keeps the board nice and clear of wiring but makes tight fits like this uncomfortable.
You can see in my second spaghetti picture that I have the optical drive mounted (Samsung SH-B123L, the cheapest blu-ray I could lay my hands on) in the centre, on the left my 1TB Western Digital Green Power drive and on the right the SSD drive. There would be no problem mounting two drives on the left although the upper one might not have a lot of airflow which if it is getting high levels of activity might be a problem (as an HTPC that's unlikely so not a worry).
On the right the mount is different (2 different mounts designed, they fit each others space perfectly because mine were mounted back to front according the to manual description) but could still accomodate two drives.
Pictured below is the almost completed build, as you can see I've managed to tidy the wires, for the most part I have cabled tied them in place and managed to keep them in place under the drives.
In its final form I have since added another 1TB drive (a cheap 1TB Hitachi, noisy & slow compared to the several years old WD Green Power) above the SSD. The solid state drive is not mounted at all, just slotted into the space below the other drive and as there will be no movement and there is plenty of wire it will stay in place and there will still be airflow between it and the Hitachi drive.
Another feature to note in this picture is the appearance of the Asus processor fan - supplied with the board I've read several reviews where people are leaving it off to get silent operation from the board. My experience was that the CPU temperature rapidly rose to 45 degrees C, not worrying in its own right but given how fast it got there and how warm to the touch the heatsink got I decided to mount the fan to see would happen. It feels like a cheap and nasty little fan and airflow seems minimal, further there are no clear mounting points on the heat sink, just screw in between the fins.
Once in place though I was pleasantly surprise that the fan is effectively silent (as quite as the Noctua fans I had in low noise mode in the Silverstone box) and temperatures are yet to exceed 32c so works great and has stayed in place every since.