Hi Sylvia,
Thanks for answering...
We found same conclusion...
We restore OS with Acronis and somehow, (maybe because we restore multiple volumes on 2 both disks) we got boot record on both our drives.
mSata is our boot choice, and OS actually restored on mSATA drive, on SSD drive we restored only users data, but from unknown reason SSD got boot record.
Anyway, our mSATA is enumerated as Dev 3, and SSD as Dev 0, and our primary boot disk is mSATA (dev 3). After power loss BIOS forget which drive was set as primary boot drive.
And this is happen ONLY on power loss, NOT just reboot! You need to remove power cord from NUC!
This is definitely a bug u BIOS, but happens only when you have two hard disks both with boot record and you have OS only on drive (dev) with higher number.