Okay, I've gotta chime in here and bring something bigly up. If you have the "fusion drive" version - it's actually 2 drives. The this guide only covers the replacement of the top, easy to get to one. You'll have to go deeper into the rabbit hole if you want to replace the other, or spare, or spinner drive.

It wasn't until I had to change out my IR sensor that I realized I had a drive just $@$*!&%& there... I used the "IR Sensor Guide" to get the job done:

Mac mini Late 2012 IR Sensor Replacement

I guess you can use the "Dual HD Kit Guide" if you want to "upgrade" or "install" the "kit."

Installing Mac Mini Late 2012 Dual Hard Drive Kit

I'm about to embark on this journey. lol

Messed up the pins that go onto the board because I forgot to unpin it when upgrading the HD to an SSD.

Computer works fine, but no infrared, which I never use anyway, and no indicator light.

COMPUTER MUST BE PERFECT so I ordered the part from powerbookmedic.