I have a Hitachi  mini usb 'simple drive' that has an auxiliary power input jack (circled).

But I don't know what voltage is required, probably 5v or 12v.

Can anyone help?

Most likely 5V, as it is an auxiliary power supply only, and USB is 5V.
I have two similar units, and mini USB ( as opposed to micro ) can usually supply enough juice to operate.
I don't even know where my auxiliary power units are; I don't think I've ever used them.

It is 12V.

https://www.amazon.com/Hitachi-XL2000-2-0TB-2TB-SB2D-020-1HA/dp/B00G7JC8ZS/ref=sr_1_fkmr0_1?s=electronics&ie=UTF8&qid=1545190492&sr=1-1-fkmr0&keywords=hitachi+0S00214

5 hours ago, MigL said:

Most likely 5V, as it is an auxiliary power supply only, and USB is 5V.

USB (v2.0 standard), directly from computer, can give device just 500 mA per port.

https://en.wikipedia.org/wiki/USB#Power

0.5 A * 5 V = 2.5 Watts per port.

 

"Hard Drive Power Consumption

Solid State Drive SSD 0.6 to 2.8 W

2.5" Hard Disk Drive HDD 0.7 to 3 W

3.5" Hard Disk Drive HDD 6.5 to 9 W"

 

USB (v3.0 standard) can give up to 900 mA per port.

https://en.wikipedia.org/wiki/USB_3.0#Power_and_charging

 

 

Edited December 19, 20187 yr by Sensei

Sorry, my mistake.
I understood it to be an external optical drive, not a hard drive ( I was in a hurry )

External mechanical hard drives come in two types, 2.5 inch, which can be powered by a USB port as they require only 5V, and 3.5 inch which require both 5V and 12V, so an external 12V supply is mandatory.

So, yeah, you'll need a 12V adapter at about 0.7 amp ( it just drives the motors, not the logic circuits )
Again sorry for my mistake; hope you didn't go out and buy a 5V adapter.

Incidentally when I upgraded all my laptops to SSDs I put all their mechanical hard drives in external enclosures ( less than $10 apiece, SATA to USB3, from China, through EBay ).
I now have 9 of them ranging in size from 500GB to 2TB.
Plus 3 of the 3.5 inch externals ( all 2TB ) but I'll be damned if I can find the power adapters for them   .