I stopped syncing a folder in one pc and then deleted it but the folder is deleted from the ubuntu one cloud storage as well, what went wrong?
Right-clicking on a synchronized folder in Nautilus, and choosing "Stop synchronizing on Ubuntu One" will result in the folder and its contents being removed from the server. It doesn't mean "only remove the connection for this computer, to this folder."
If you wish to unsubscribe a synchronized folder on a computer, you can do so by opening the Ubuntu One control panel application from the launcher, and unchecking the box under the 'Sync Locally?' column on the Folders tab, for the folder you wish to no longer sync to the local machine. Or, you can also use the