After upgrade to kde 4.9.2 in chakra, after usb partition is mounted, filewatcher will scan over all directory and thus make this partition umount-able for a while. Removable media setting in nepomuk is set to "ignore all removable media". Reproducible: Always
I have also experienced this problem in my Gentoo Linux with KDE 4.9.3. It is pretty annoying.
The same here in Arch Linux with kde 4.9.3.
usb drive added to fstab with options user,noauto. After mounting the drive, neither "user" nor "su" can umount it. I have to go leave KDE and umount it from terminal. Disabling nepomuk solves the problem. Umounting an usb drive in KDE is then possible. A very annoying bug. System OpenSuse 12.2 64bit, KDE 4.9
*** This bug has been marked as a duplicate of bug 304943 ***