Coolermaster.
I've been testing it all evening. I might be wrong about power, it did seem strange. As you say it shouldn't need much more for a few hdd.
I've put all the HDD back in and no crash yet.
I missed one other significant difference to earlier tests. This time when I installed the AMD catalyst drivers, only the driver itself worked, all the other parts of catalyst e.g. control panel etc failed.
So it may (more likely) be something the full catalyst packages install does, e.g. default overclocking?
Now I've got all drives back in, but only the gpu driver installed, it seems to be working. Only found that due to some other bug where catalyst starts failing to install stuff..