I did some messing about with a Pi and some hard drives, and it was all good.
First, I attached a dual-Sata HDD docking station to the Pi (via a powered hub). Immediately, I was able to see the two drives on the Pi as sda and sdb.
So then I tried adding a second docking station - this one allows one Pata IDE drive and two Sata drives. That showed up as sdc (for the device itself, which also lets you read all sorts of memory cards, so I'm guessing that sdc was the card. Also, the Pata IDE showed up as sdd, and the two Sata drives were sde and sdf. Notice, I didn't install any software, I just plugged them in and it "just worked". The two-drive device used 17 watts, the three-drive used 28 watts.
A couple of deliveries today - I'd ordered a bunch of USB-hard drive interface cables, which come in at just under £3. There's no power supply, no Sata cable and no Sata power cable, but I knew that. So I fired up an old PC power supply, to give me the power for the hard drives. I wanted to see if I could use both the Pata and the Sata port at once, giving me two drives. Yes - it worked. And the two drives (plus the overhead for the power supply) was 29 watts.
So then I wanted to see if I could add another drive using another interface cable. You guessed it - it "just worked", three drives on the two cables, pulling 36 watts.
Huh. So I added two more interface cables, each with a Sata drive. And they just worked. Five drives pulls 50 watts. It's 7 watts per drive, plus 15 watts overhead for the power supply. That particular power supply is a measly 200 watts, so I could put a couple of dozen drives on it, and I rather think that the Pi will happily handle that.
I also had a look at one of the seven-port USB hubs I got for the Pi Project; I'd plugged it in and it didn't work, so I put it aside for now. On closer inspection, I noticed that the soldered joint to the power wasn't working, so I soldered that, and now the hub works fine. And another of the seven-port hubs had exactly the same problem. I thnik the problem is, that part of it is hald-soldered, and anything done by hand has a higher failure rate than anything machine-made. It's the same problem, I think, as the large smoothing capacitor on the pi.
I've ordered a couple of 10-port powered USB hubs, about £4 each; if I'm going to add a lot of hard drives to a Pi, I'll need those.