Spin-up time
The time a spinning device needs from power-on to full speed and ready-to-read — trivial for one drive, a power problem for a hundred.
A hard drive has to spin its platters up to operating speed — 5,400 to 15,000 rpm — before it can read anything, which takes 4 to 15 seconds. Start-up current is the drive's highest: the spindle motor draws most just as it overcomes inertia.
That is why disk shelves use staggered spin-up (SATA/SAS PUIS, "power-up in standby"): drives start one at a time on command instead of all at once, or a full rack would trip the power supply. The same wait is the downside of power-saving idle — parking the heads in the landing zone and stopping the platter saves watts but adds spin-up latency to the next seek.