Usually, attaching a regular USB storage (usbstick, hard disk, mp3 player...) to a recent version of GNU/Linux running a common desktop environment like Gnome, KDE or XFCE - you are presented with some sort of automatism, allowing one to mount the newly found device.
In my case, an external drive as well as an MP3 player just never showed up. hmpf.
[SOLUTION]
It seems that they needed to be "bumped" once to introduce themselves to the system. So can you "bump" a USB device? By executing the "lsusb" command, that should list all USB devices attached to your computer:
The first output of lsusb won't show the device, but it initiates a refresh and thus causes the device to appear.1) Attach USB storage device
2) wait a few seconds and then type "lsusb" into the commandline.
In my case it's probably caused by a faulty USB hub that behaves a bit strange...