I'll chime in here... I've seen something similar (on my 2862ac)
Since purchasing the device 10 months ago I had an issue where syslog to USB would not automatically restart after a router restart - I'd have to toggle it off and on to kick it into life). Annoying as I also have an issue with uncommanded restarts ('sys ver dbg' shows nothing and nothing shows on logs to syslog server). THEN with 3.9.0_BT, it appeared that the syslog to USB issue MIGHT be resolved. It was certainly restarting (most of the time?). I left my syslog server running anyway. [I have reviewed the unexpected restarts article - nothing explains]
A few days ago I had another unexpected restart. I forgot to check that syslog to USB was working immediately thereafter (I was getting more relaxed that the issue was fixed).
Then yesterday, I realised that the USB DISK (USB Device status - disk tab and USB file explorer), was showing as "disconnected" in the UI. The USB LED was on, the UI showed the USB port highlighted in green on the graphic and interface list. Re-inserting the USB stick did not help. Reviewing my syslog server showed the USB stick being inserted and identified correctly. The were no messages about attempts to mount the filesystem or failures doing so. I have not had time to investigate more deeply yet.
The stick in use was the on that I'd used a lot before and was in a port where it had been for a month or so (and had been working within the parameters described above).
