If you have upgraded to KDE SC 4.11 on Linux Mint 15 through the backports, you may run into the issue that removable storage devices are no longer detected by the device notifier or in Dolphin. To fix this, remove the package udisks and install the package udisks2. That should fix it.
(source: http://www.reddit.com/r/kde/comments/1k ... de_to_411/)
Made this topic sticky for a while, as I guess more users may run into this.