THIS IS A VERY SERIOUS PROBLEM WITH UBUNTU ONE
I have had 12.4 working since May 2012. This month (Nov 2012) I noticed that a minute or two after logging in, the disk access light stayed on solid. It was permanent. The only way out was to pull the plug on the machine. The only access I could get to the disk was a few milliseconds per minute.
One time, I actually managed to open a terminal and run iotop. With the disk light hard on, iotop registered only a trivial amount of disk access. Whatever was hogging the disk was below the level that iotop could see.
I had no background apps running. I could not use nautilus as it stayed greyed out because it couldn't access the disk. I certainly could not open the unity dash.
The disk was not used excessively either when nobody was logged in or when the only other user on the system logged in first without me having logged in. If I logged in first then the other user had the same problem.
I could not use any tools to investigate the problem while logged in to my area. Then I realised that the only difference between me and the other user was that I had ubuntu one installed.
I managed to get through to the System Monitor at one point after waiting for ages and ages for it to open. I tried to kill the ubuntuone syncdaemon but I couldn't. When I tried the marking shifted to the next process down the list every time I tried. The daemon seems to have some kind of defence against being killed.
I tried vainly to uninstall ubuntu one but I could never get the software centre open before the disk reached a solid 100% access. I managed to open a terminal to use apt-get to purge ubuntu one. But after several hours it hadn't even managed to read the DB.
Fortunately, a couple of days before, I had made the other user an administrator also. I was finally able to purge ubuntu one from the system with apt-get in the other user's area. Problem solved.
I think that, as administrator, I should be able to have control of what does or does not run in my machine. This was a rogue program over which I had no control. It required some ingenuity and a lot of luck to finally eradicate it from my machine. I think it is an extremely dangerous program. No program should be able to exclude the user completely from the use of the machine.