can you fully exit it, or is it always it the background?
I think I will install it if I need to do a firmware update. Is their anyway of telling if the firmware is up to date without magician installed?
Are you worried about installing the Magician software ?
I have read reports of a few problems with it, but I`ve never experienced any issues having carried out about 20 installs.
I thought it was quite bloated, but its not bad at all, as Ive just installed it. Most of the options are grayed out atm as the sammy isnt installed yet.

Does updating the firmware on the 840 erase all data ?
No all data will still be on the drive, its just like updating the bios on your motherboard.