i own a galaxy S2 and some day i couldn’t connect to my pc anymore via usb. Windows/Linux, different machines, nothing worked.
On my hunt throught the internet i found some forums stating that there are problems known with the usb port of the S2, so i ordered a new one and replaced it, but that did not help either.
Finally i found this blog post: http://www.bitblokes.de/2012/04/losung-samsung-galaxy-s2-verbindet-sich-nicht-als-massenspeicher/
There it is said you have to enter the following number: *#7284# and then change from MODEM to PDA or maybe from PDA to MODEM and back.
That’s it, it does work again now. Seems to be a bug within the firmware. A pity i have not found it earlier and had to buy a new usb port w/o really having the need to.