Chosen Solution
Why is my late 2012 mac mini only showing the 121 gb flash storage and not my larger disk in disk utilities?
It’s possible the internal SATA cable connecting the internal HDD to the logic board has become disconnected. I’ve seen this happen accidentally on this model (it may be a result of transporting the machine or moving it around physically). Inspect the hard drive cable connection shown in Step 12 of this guide. If it is loose or coming loose, you can reseat it and use some anti-static tape to secure it in place. You may also want to inspect the other side of the cable, where it plugs into the hard drive. If that end is loose I would also recommend reseating it and securing with anti-static tape. Hope this helps!
