Well, at least in the US, there really aren't any good commercial HDD blocks, AFAIK. Most of the ones that are available are Aluminum. However making a reasonably good performing drive block setup is probably the easiest DIY block project there is. The work is simple, and relatively few things are needed in the way of tools.
Keep in mind that even the worst heating hard drives put out relatively little heat, the problem with drives comes with the long heat soak that builds up if the heat isn't dissipated. Thus almost any level of supplemental cooling will do for a drive.
The first question is what part of the drive to cool, I've seen designs that worked on the top and / or bottom (top is cover plate, bottom is PC board) and others that work on the sides. all reports are that either approach will work fine if done properly, the choice should be based on the mounting desired and ease of build. Either type of cooler is going to increase the size of the drive mount, obviously top / bottom coolers will make your drives taller, and side coolers will make them wider. Space must also be allowed for plumbing connections, to the extent possible, I like to make my hookups on the end opposite the cables just to space things out a bit.
Most of the heat in a hard drive comes from the air friction of the platters inside the drive housing, and whatever area of the drive is cooled, the remaining heat will be rapidly transferred to it by convection / conduction. I have even heard of people sticking a GPU or NB block on a handy area of the drive and getting good results. The PC board on a drive doesn't contain any major heat generating parts, and shouldn't need any extra or specific cooling.
The technique I suggest is to use copper plates (Approx 1/8" - 1/16" (2-4mm)) and 3/8" or 1/2" copper tubing soldered together and connected with standard copper plumbing fittings.
If making a top / bottom cooler, it may be good to use 1/4" tubing if you don't mind the increased restriction.
Side coolers are very simple and work well with multiple drives, figure out how many drives you want to cool, and then make two copper plates the appropriate height, and with holes drilled for mounting the drives (use a 3.5" drive bay for a template) In between the rows of drive mounting holes, run lengths of copper tube soldered to the plates, make bends as needed with pipe fittings, and connect the two sides with short lengths of appropriate sized poly or Tygon tubing (whatever you use in the rest of the system) If you watch your overall width, you can mount the assembly in a 5.25" drive bay. I've also seen setups where the person used the side coolers to replace a 3.5" drive cage (this was more advanced metal working)
Top / bottom coolers are made in a similar way, only the plates would be drive sized. If cooling multiple drives, an alternating layer technique is useful, with one cooler on the top and bottom of the stack, and an additional cooler between each pair of drives. The between drive coolers should have plates on each side. The hard part is figuring how to fasten the stack of drives and coolers together.
The big question is, should I just suspend the HDD(s) in the path of some airflow, and avoid the restriction in the watercooling loop? Assuming I'm using an MCP600 with a Thermochill HE120.3 radiator, CPU, GPU & NB blocks already, would one of these just kill what pathetic flow rate I had left?
With that pump, your flow rate shouldn't be all that pathetic if you plumb everything else right. I would use a series / parallel setup, so that all the coolant went through the rad and the CPU block, then I would split the flow and send half to the GPU and the other half to the HDDs, NB, and any other devices getting WC'd. That should give you plenty of flow. Alternatively if you just left the drives hanging in the airflow coming off those three rad fans you should be OK without WC'ing the drives.
If I go ahead with the V1100 plans detailed in My first SPCR thread*, keeping the HDD area and PSU seperate from the rest of the case airflow, would it still be worth using a drive watercooler just for the noise reduction?
In and of themselves, an HDD cooler won't do much for noise, it's biggest advantage is that it will allow the drives + cooler assembly to be encased in some form of noise deadening enclosure w/o cooking the drive. Suspending the enclosure might work, but would be a challenge because of the added bulk / weight, plus the need to deal with plumbing connections. Rubber grommets are out, the drives need to make good mechanical contact with the plates.
Finally, if I give up on the V1100 and use a standard case with the radiator on a bottom intake, am I better off using a watercooled PSU and a rear case fan to get airflow across the other warm components? Or am I better off just getting a naturally quiet aircooled PSU, and maybe? just using that and the positive pressure from the three fan radiator to produce adequate airflow?
Assuming all three fans are blowing into the case, the positive pressure should be MORE than enough to ventilate the case, I would just use a quiet aircooled PSU and skip the rear exhaust fan.