Your Crucial 120 GB mSATA SSD CT120M500SSD3 should work. It is listed as
Tested on DC3217BY NUC but not on my D34010WYK (4th generation core i3)
Tomorrow morning I'll try to update BIOS and see what happens, (1 AM now here asleeeeep).
Will comment tomorrow.
Or the CRUCIAL M500 120GB is defective or NOT COMPATIBLE with intel NUC's.
I have update the BIOS to 2/10/2014 mine was something from 2013 and continue without detecting de mSATA SSD.
I have bought a new intel 525 Series 90 GB mSATA SSDMCEAC090B301 listed on the compatibility page, I'll have it on monday. Let's see on monday if this one work.
Nobody has answered if there's any option to check in the BIOS about mSATA ssd's or is as simple to put the SSD and automatically be recognized.
In the BIOS are Legacy Boot and/or UEFI Boot enabled? It shouldn't matter but we should still cover the bases.
Once the mSATA is installed if should show up in the BIOS. I don't think there's an option to turn the port off in the BIOS, but try resetting the BIOS to defaults.
You might not recognize the mSATA. In my BIOS .18 version mSATA is described as "SATA Port 3" when in UEFI.
If you click on Legacy the mSATA is described by brand and capability.
When I installed Windows 8 the Sata Port 3 was #1 in the boot order in UEFI. I clicked on Legacy spotted the USB flash drive with the W8 key and clicked on it. Everything after that was automatic.
When I run Linux live I click on Legacy and then the applicable USB flash drive.
Otherwise for everyday operation the BIOS is set in UEFI.