In a post on electronics.stackexchange.com (now deleted because off topic there) was explained, in details, the reason why is not advisable to disable an hard disk (pc not turned on, of course) simply detaching it from power supply unit. Please see below some quotes.
Now, by chance, I have changed upon racks for internal hard disk endowed with power switch "included for power saving or safe removal" like, as example, this one.
Therefore I wonder if what was elucidated before still holds here, or the tray is equipped with some device to resolve all the problems mentioned in that post.
As far as I can suppose at present moment, it seems unlikely that the switch breaks all four cables of the power supply unit.
I would not break the black cable, it leaves 7v across the 5v and 12v inputs, and with no ground, who knows what could happen. It may even send current through the data I/O and damage those. Even if there's no destruction mechanism in this state, I doubt it would come up cleanly when the connection was restored. Invest in a 2 pole switch, and break the red and yellow.
Next to @Neil_UK advise, do still use 2 black wires. Without knowing the reason for two wires do still question: why would the manufacturer use 2 black wires (and make more production costs) if 1 would be enough? (Manufacturer's reason could be current rating, splitting power ground and signal ground, etc).
Even if you switched all of the power lines you would still have the drive's I/O signals connected to a powered-up motherboard. There is a good reason why drives are controlled the way they are, with a combination of hardware and software.
