You want it to be 12 Volts and lower than 1.0 Ampere by a lot since that power is what is powering the drive. If the fan drew 1.5 or 1.0 Amperes there would not be enough power left over for the drive.
I have an Iomega 1T external HD with a Seagate in it. It's about a year old. It's just about full, and gets alot of use. Some times while using it the aluminum case gets so hot I can barely touch it. I also forget it on alot, but it seems to have an automatic rest mode. It doesn't seem to get as hot when left on but not in use. One way or the other heat is bad.
What I want to do is drill a large hole in the aluminum case, maybe 1-2 inches, and permanently mount a 50mm fan on the side. I want to splice into the power supply wire going to the unit. The power supply is putting out 12volts and 1.5amps. I can't find a 1.5 amp fan. does it matter as long as it's 12 volts?