I have a 2012 MacMini with two hard drives. The hard drive containing the operating system has died. Which of the two drives in the MacMini is contains the operating system?

Open Disk Utility, select the Drive, Control (right) click on the highlighted Drive and select Get Info.

Optionally go to: Apple > About This Mac > System Profiler > select SATA/SATA Express in the Hardware section. Then if you select each of the listed drives on the Serial-ATA Device Tree, it will tell you which bay that drive is in.

Should you be able to use one of the possible choices in macOS Recovery

to access either or both internal hard drives, you could use the Utilities from

Recovery startup to tell which hard drive has what content; and also try to

see if you could use Recovery to see if the failed hard drive could be repaired.

• About macOS Recovery - Apple Support:

https://support.apple.en-us com/HT201314

There are specific details in what key combinations to use for different results.

And additional links within above article offer more detailed choices...

Per instructions in above page: "...The re-install macOS utility installs different

versions of macOS depending on key combinations you used while starting up.

For details see How to reinstall macOS - Apple Support."

If you have a Time Machine backup, that may be useful in restoring the system

to either a new replacement drive, (outside the Mac, in an enclosure; or by wire

USB drive adapter without enclosure) such as this example from newertech:

NewerTech Universal Drive Adapter USB Bare Drive... at MacSales.com

And if that's all that failed, a restored system on replacement drive could then

be installed into the Mac; this can be tested prior to take-apart of the Mac. A

hard drive replacement for original in this manner should be able to run Mac

by use of the universal drive adapter USB wire kit (it has power supply, etc.)

There may be another issue behind the Mac failure; perhaps different hardware.

What symptoms does the Mac mini exhibit? ~ It may need to be tested to see if

any original components on the logic board or perhaps power supply(?) failed.

Perhaps an authorized Apple service provider may be able diagnose other issues.

Good luck in any event...! 🙂

If your Mac Mini has 'dual factory hard disk' drives, the original default location

of the boot drive where original system was installed is the 'Lower' one... when

the Mini Mac is in a running position. Or the bottom of two, in this configuration.

My i7 2.3GHz server shipped with dual 1-TB 5400-RPM rotational HDDs stock

and the system(s) are on the default hard drive location; HDD-2 has no content.

But the System Information (OS X 10.11.6) does not show 'a diagram' of location.

Also either hard drive location could have been used as boot. But the default HDD

within my Late 2012 model, appears to be the bottom of the two, nearest access.

An iFixit guide for Mac mini desktop model may help; be sure & identify it correctly.

{During an upgrade to the Mac OS X, one can choose either drive as install location

and in addition, put a second full system so as to facilitate dual-boot capable Mac.}

Good luck & happy trails! 🙂

It is a little more work and requires another Mac, but I would pull both drives and test them in an enclosure, dock or on a SATA to USB adapter to figure out which on is bad. In addition you should be able to install OS X or macOS on the good drive in the Mac Mini (like K Shaffer suggest's) or out while testing them externally with the other Mac.