Moulinette not accessible

I have a usb disk mounted as my main owncloud data folder.
The disk seems not reachable anymore it gives the following error: Erreur d’entrée/sortie
but what is more strange : I can no more access the web based moulinette: I get the following error: L’API ne répond pas (erreur : 0 timeout)
Is there any logs, I should look at ?
Mean while I will try reboot the server

after reboot I can connect to the moulinette again and the usb disk is
Disk failure ? BUG from one of the service? Disk went in sleep mode (thought it would mean that it took it a few weeks before getting into sleep mode) ? Hacker ?
Note that the NUC was really hot and though I could not access the moulinette or owncloud or the disk, the server was still working (sso page was functional)
any idea where to look
especially if sleep mode does not work would need to have a solution, or if someone got inside the system.