I have had that issue too. This is my second m4 in less than a year. First one kept dismounting SD card - tried multiple cards and same issue - worked fine on earlier phone and my husbands when I borrowed his to test. Then it started showing upgrading android on restart - but most annoying thing was it was loosing cellphone reception - going from full bars to emergency mode only - and only at one location where other phones were fine. So Sony replaced my phone.
Unfortunately my new one has just started to do this again. Doing factory reset appears to have got rid of issue with android updating on most recent reboot - but still regularly lossing cell reception.
