Modifying 12 v Power cable to Molex

It might be a branch of the voltagesupply that is only reserved for heavy-use with devices like motordrives and if that branch is just a simple supply with a bridge-recitifer and thereafter just balanced with a large capacitor you just might have seen the "bouncing" of such an nearly unregulated powerbranch ..... the voltage drops dramaticaly at the "switch-on" and "spin up to speed" commands of the motor that draws initialy very much current and causes that "bounce" ( i.e. dropdown )and comes back to normal voltage when the motor has reached to regular speed and draws then less current.... if replaced with a very rigid regulated powersupply that heavy balance and load might cause the reguletor to shut off the voltage because it believes a shortcut to be in that branch..... the solution will be to reduce the feeedback and the trim of the voltage-shortcut-regulation to be less imperativ and more tolorant and to permit the draw of more voltage within that branch.... there is probably the need to do some more math on that circuitbranch.... specially at the circuitprotection feedback branch....

Power supplies with no load usually read higher than normal voltages. If it had a load and was reading 7.35v at the 5v lead, then I'd be concerned

Can't say about your particular hard drive but many hard drives won't spin up unless accessed by a computer. I have a USB to ata adapter that came with a power supply. While some drives spin up as soon as power is supplied, most drives don't spin up until I plug the USB into a running computer

Most 3.5" IDE or SATA drives require both 12V and 5V in order to operate properly. Just 12V usually won't do it.

They all require 12v + 5v 12v to drive the motor, and 5v to power the electronics