I've just had a similar issue with my m4 256GB mSATA (01MG) drive.
My block was recently experiencing frequent power outages, and after one of them, the drive would no longer be recognized at POST.
It would hang at POST for ~30 seconds longer than usual with it plugged it, but not detect it in the end.
Booting to Hiren's and trying to reset its Active status didn't work either, since it wouldn't show up neither in Acronis nor in GParted, both in AHCI and IDE modes.
What solved it, surprisingly, was another power outage in the middle of me trying to figure it all out...
Since the power came back on, it's detected fine again - but my sense of confidence in this drive is somewhat reduced now.