If I want to power up multiple hard drives do I need to connect the power cable from the psu to each hard drive, or on just one and then a sata cable on the others? Thanks.
Each drive needs it's own power. That's why there are two connectors on each drive. " . . do I need to connect the power cable from the psu to each hard drive. . ." YES.
Yes, each drive must have its own power cable. Occasionally, but not often, you might come across drives like this one: Note that this has a S-ATA power & data connection as normal on the left, and an old Molex connector on the right. If you have a drive like this NEVER connect both power connectors at once.
Neat. Haven't seen one of those yet, but then I have a s**tload of molex/SATA power adapters lying around so I haven't paid attention.
I've only encountered it on WD's older drives such as the WD2500JD here (which I have two of) and the WD360GD Raptor (I believe it also applies to the 74GB versions)
I think this was for the OEM's...so they could use up their stockpile of old power supplies. I have three old WD 40GB SATA 1.5 drives that have these molex inputs, all came out of old Dell machines.
No, it was just standard practice from WD at the time - all the drives I bought with connectors like this were retail.