So I have been just living with this for a while now, whenever I mount my sd card as USB mass storage and then unmount it, the phone will reboot. It will get to the unlock screen and then reboot again. It does this a few times in a row and then all is fine, until I unmount the SD card again. It is really starting to bug me.
I could always just wipe and install the ROM again (Smoked Glass) but was hoping there was a real solution.. USB diagnostic mode works fine, it is only until I either unplug the USB cable from a mounted SD card or unmount the SD card.
For a while I could get by with rebooting to recovery to work with the SD card but now that too causes the rebooting problem after leaving recovery.
I thought it was SD related so I just took out the SD card after it rebooted on me and it did the same thing - rebooted twice or thrice and then was fine again.