Garmin eTrex 30 unbrick?

I hava a garmin etrex30 here. It's very strange. I cured 4 different garmin devices in the past.

Now I have a Garmin eTrex30 and it is absolute strange. On startup it showed just the garmin logo and the small text showing etrex30, Copyright... Then it's frozen. It stucks in this state.
I produced cure and original firmwares and installed different versions. Up to FW-Version 4.1 (I didn't try 4.2 to 4.8, the actual one is 4.9) the eTrex started. It was detected by Garmin express and I successfully installed language-files. Then I tried to install updates to 4.9. It stucks at boot process.

So, next step. I formatted the internal storage with LinuxMint. Of course I know, this might cause problems. And: I could boot the FW 4.9. But the eTrex could'nt either find Data on the Storage nor could it write to it ("database error"). Yes, that's because I formatted it with Linux. Afterwards I formatted it with windows (not quick format). Still the same. So I used RMPrepUSB. And - it stucks again.

So I don't know, what to do... Maybe some of the really experienced users have some advice for me Thanks.