Do you still have this problem? If yes, it could be the same as KDE Bug 293906 - KDE device notifier does not show USB devices after umounting
I am also experiencing this effect in KDE 4.8.3, but only for pen drives; the icons of external USB HDDs remain in the list of Available Devices, with a tooltip 'Click to mount this device', and so I can click on the icon and then select 'Open with File Manager' to re-mount the device.
Notice that, in Device Notifier, the icon of a USB pen drive is different to the icon of an external USB HDD. So KDE knows they are different devices. I suspect that the code in KDE has been written to assume that, if you click on the icon to remove a pen drive (the tooltip 'Click to safely remove this device'), then you are going to remove it ("why else", the developer must have thought, "would you click on the arrow icon to remove the pen drive?"). Isn't it a bit like the functionality in Windows? Doesn't the pen drive icon disappear from the Windows pop-up window if you select 'Safely Remove This Device'? I can't recall now.
Anyway, I wonder if there is some D-Bus (or udev, or udisks) rule somewhere that the user could edit to change this behaviour for USB pen drives specifically.