I have a problem. Googling shows that other people have similar problems, but unfortunately the solutions that worked for them didn't work so me, so I'm going to have to ask myself.
I have a (rooted) Samsung GT-S5830i ("Samsung Galaxy Ace") running Android 2.3.6, and when I plug it into my Ubuntu 12.04 desktop using the usual kind of usb-to-micro_usb lead...nothing seems to happen.
The command (on the Ubuntu machine)
gives the same output whether or not the phone is plugged in. In particular the phone does not seem to be being seen by the computer at all.
Similarly the phone does not give me the usual "you've just been plugged into a computer" symbol in the toolbar at the top, and in particular I am not being asked by the phone at all whether I'd like to connect via USB -- the phone offers me no new dialogues or options after being connected; it just sits there charging [in particular it's not the case that e.g. the wire is completely broken, or the port is completely dead; I've tried more than one port on the computer anyway].
I have tried more than one USB connector wire. I have tried more than one USB port on the computer. I don't know where to start debugging this issue.
I can access the files on the phone using various other methods, for example via Dropbox, Software Data Cable, bluetooth etc. The issue is simply that this particular method doesn't work and I don't know why. [Note added later on: I suspect this assertion is actually false -- I think that what I should have said was "yesterday I could access the files on the phone using various other methods; today I couldn't access them on my ubuntu machine, but it hadn't occurred to me that in fact I couldn't access them on any machine at all via any method."]