Mac Mini internal disk drive doesn’t show in Disk Utility nor Terminal Anymore

Mac Mini doesn’t boot from internal SSD however, it successfully boots from the same SSD when it is externally mounted via a USB adapter.

 My today I tried to partition my boot drive on my 2014 Mac Mini. I successfully erased and partitioned the drive. The primary partition was an APFS format. The secondary repetition was in Mac Extended Journal and finally I had an exFat partition which I would use for windows. After I notice that the computer was going to try to install Sierra instead of Yosemite (I’m running an old copy of Logic Studio), I stopped the installation and returned to the Disk Utility. There I noticed that my boot drive was no longer visible. So tried checking the Terminal and I got no info back on the SSD.

I figured I should reformat the drive, but only as Mac Extended Journal. So I took the SDD out from the computer and placed it in external enclosure to format it with another computer. I also flashed a copy of Yosemite on a USB stick. 

I thought I was ready to go and placed the SSD back into the Mac Mini and also inserted the USB stick onto the Mac Mini. I turned the Mac Mini on and pressed and held the Option key and when my drives showed up only the USB was visible. I started the computer again and went into Recovery Mode, where Disk Utility and Terminal were both not able to see the internal SSD yet could see the external USB perfectly fine. 

I turned the Mac Mini off, removed the internal SSD and placed it back into the external enclosure. I now connected the external SSD via USB as well as the USB drive with Yosemite on it. I turned on the Mac Mini and now both the SSD and the Yosemite USB stick showed up as external drives. I then was able to successfully install Yosemite on the SSD.

Once the installation was finished I ran a few Logic sessions and to make sure everything was as running smoothly. I then turned off the Mac Mini and removed the SSD from its external enclosure. I placed the SSD back into the Mac Mini and turned it on and again I was meet with the Folder with a Question mark the support.apple.com/mac/startup message. 

At this point I turned off the computer, placed the Yosemite USB back into a port so I could have picture proof of what was happening. I turned the Mac Mini back on and went into recovery mode. There again I saw no SSD and only the externally mounted USB Yosemite drive.

Does anybody have any idea how to fix this? Or could anyone recommend someone who might have some knowledge on why this is happening?

I’m fairly new to using Mac products so I any advice would be greatly appreciated.