After a PC connection (mass storage mode under Win 7 with Garmin Express, GSAK and Explorer) when I synched the device, downloaded tracks (for heaven's sake!), uploaded a GGZ with geocaches and finally ejected the GPSr properly, the O750t (FW 4.00) refused to boot. It stopped at the Garmin logo (before the copyright information appears) and didn't react to any button. I removed batteries and SD card, I tried to enter the enforced mass storage mode with connected USB, I tried a soft reset but nothing helped.
The Garmin service replaced my two years old unit. (Contact with the service by phone was perfect and the new O750t arrived 5 days after I sent the bricked one to them. Perfect!)
Unfortunately the first thing that the new unit did after I configured WLAN was an automatic update to FW 4.00 again (which I don't "like" because of the chirp bug).
Three weeks later the same fault! After PC connection (and proper eject) the unit refused to boot. Removal of SD card and trying a soft reset didn't help. I alreay contacted the Garmin support again (this time by email). But then I tried to enter the enforced mass storage mode (this time without battieries). As before when connecting USB my PC gave its "device connected" sound (but without already having access to the "drive"). I continued to press the power-on button. After about 20s the PC gave its "device disconnected" sound (same as with my former unit) but then after the 30s for the soft reset the unit sounded a "beep" and connected to the PC again. The soft reset worked!
As I had to reconfigure the reset GPSr anyway, I decided to fall back to FW 3.90.
Here are some evidents I noticed:
- both units ran with FW 4.00
- both times another external drive was connected simulateously to the PC (but not for the first time)
- both times my backup software (Acronis True Image) ran shortly before I connected the unit
I am quite "frightened" because I do not know if I did something wrong or what the root cause for the failure might be.