As above. An HDU spinning up will consume about 20-25W each. 4 hard drives = 100W on them alone. Normal operation is only usually about 10-15W however (SATA drives are usually slightly higher in terms of power consumption).rpstewart said:The problem is likely to be how many that you can spin up at once rather than how many can be run in normal operation, the startup current of HDDs is several times more than their normal operating load. It therefore depends how the disks are going to be attached to the system, for normal motherboard attachment you could run into problems with only 4 or so (depending on CPU etc) but if you're going for external card attachment then with staggered spin up you might get away with more.