I have updated the firmware(EXT0DB6Q) with Magician 4.6 under Win7 on my 840EVO 120GB.
(I have dual boot system, Linux Mint 17.1 on 840EVO, Win7 - just for some windows programs - on HDD)
Firmware update was successful everything seems to work, under Win7.
When I started Linux Mint I got a lot of file system errors, freezes, and a system that couldn't boot up.
I've find out the problem is with TRIM. (TRIM seems to be OK under Win7)
The SSD become become totally unstable after the TRIM command (fstrim) on Linux Mint 17.1.
I've got massive I/O errors.
I had to disable the scheduled(on every boot) TRIM functionality under Linux Mint.
After that, Mint was booted normally, now 840EVO seems to be stable (no sign of data loss).
TRIM was fully functional with the previous EXT0CB6Q firmware on Linux Mint 17.1.