Don't know if you're still looking for answers regarding the MDS600, or SSA70, and it apparently was previously known as, but I have one, and can answer some questions about it. (I was actually lurking in these forums to learn more about it, in case there were others who've already gone there.) I registered so that I could add my experiences to this thread.
My SSA70/MDS600 has recognized both SAS & SATA drives that I've plugged into it, in the same drawer, and at the same time. The LSI-9280-4i4e that I had connected even gave me choice of which drive to boot off of, although this is functionally 2 separate drawers of 35 drives each. You'd need a dual-port external HBA or RAID/SAS controller to use both sides at once.
I haven't measured the power usage of this, but it sounds similar to standing out on the tarmack next to a 727 with engines on idle, so it may not be for the faint-hearted, noise-wise.
I've read through the HP manuals online for these, and apparently, if yours identifies itself to the HBA as an SSA70, you may either need a firmware update, or an I/O module replacement to get it up to date, but you can't update the firmware on these without an HP controller card, and you also can't edit the zoning without a genuine HP card or HP SAS switch. I did buy an HP P800, which does work, but I don't care much for the HP RAID cards or their software interface, so I have it just in case I need to edit the features of this system.
Another caution, the manuals and unit say in multiple places that you should have all of the spots populated with either drives, or fillers or blanks, in the case of the redundant fans/power supplies/I/O modules, etc., in the back, or you risk overheating your drives. If you're buying one of these in an ebay auction (which I did without doing my homework first), try to get the seller to include the drive trays, if you can, as it will save you a bit down the road.
I haven't yet done any link testing of it, but I suspect that mixing SAS & SATA forces the entire drawer to SATA 1.5 Gb/s speeds, so if you were looking for slow, but cheap storage, this may still work for you. It will only do SAS 3G speeds if all of the drives are SAS 3G, which could get pretty expensive.
I don't have any expanders, so I can't check how it works with those, but I do have many LSI 9280-4i4e cards, a 9280-8e on the way, and various others with internal-only ports and no good way to export them.
I got lucky enough to only pay $300 for mine with all the trays, and I drove 4 hours to pick it up, rather than pay $175 in truck-freight charges, & still have to hassle with the trucking company about scheduling a residential delivery. It is fairly heavy, at 160 lbs empty, and probably close to twice that with all drives installed. It has 4 redundant hot-swap 1200 watt power supplies, and 4 hot-swap fan modules, which turn on as soon as the power cords are plugged in, whether or not the unit is turned on.
If it comes with dual I/O modules, that's all you need, unless you have a SAS switch, and are using dual-port drives since otherwise, you'll not get any benefit from doing so. The modern version of this is the D6000 storage array, which is 6G SAS all the way, and is reported to not even work with SATA drives installed. They look the same, although HP warns that the I/O modules for the D6000 will NOT work in an MDS600 or SSA70 at all.
Hope this info can be of some use to others, as I'm not sure if I should put mine up for sale, and look for a set of 3G MSA60s, or go faster, and more current, by looking for a set of D2600s from HP, although they won't hold as many drives, they're probably much more energy efficient to operate...