It's generally a "don't care" for the typical home user with one to three drives.
Look at the specifications for the drives. Unfortunately major manufacturers decreasingly break down Amps or Watts by power rail...they just provide a total. But the label on the drive itself should break it down by power rail.
The specs will tell you exactly how much the maximum
current and/or power requirements are in total or for each voltage rail...which is what you're worried about. Depending on what the drive is doing it may consume less power...as little as ~15% of the maximum.
It also depends on "family" of drive within a manufacturer. E.G. Western Digital has a "green" family that consumes maximum total
power ~5Watts, half the maximum total power of WD's Caviar Black drives.
You can either compare the amps or the watts for each voltage individually, or add the WATTS for a total Watts for a given drive. You don't add Amps from different voltage rails.
For example, for the +12V rail one might see 0.8A at +12V. That = 0.8Ax12V=9.6Watts ~10Watts on that +12V rail.
For drives that use more than one power rail (notebook drives generally use +5V only)
0.8A x 12V =9.6W at +12V rail
0.4A x 5V = 2.0W at +5V rail
= 9.6W + 2.0W = 11.6Watts TOTAL for that single drive.
Here's an example of the specs from a Samsung SATA drive. Note that "spin-up" is provided in Amps...the spindle in 3.5" drives that have +12V power is usually powered by the +12V rail. This would suggest an estimate of the instantaneous maximum
power on the +12V rail for this drive is 2.0A x +12V ~ 24Watts. Take off an roughly estimated 15% for +5V power needs, and that makes about 20Watts maximum on the +12V rail. But that lasts only for a very short time as the drive spins-up from 0 to 7200 RPMs.
edit to add:
hyperlink to drive data webpage