Ok, so it magically fixed itself?
Did you plug in your external hdd after it was started? Then it might be a udev hotplug issue, as it seems to work if it is already connected when the box boots.
No. I had checked the HDD with a different plugin other than File Browser, namely the DreamExplorer Plugin - which I installed myself not from the Downloads. All I could see then, were the contents of just one same partition four times over but not the remaining three or of a usb pen drive. And this after having installed the mountmanager plugin. Didn't know that all I needed was the File Browser Plugin from the Downloads.