1. Try another USB port.
2. Try a powered hub.
3. Try another cable.
4. Get the disk out and try other (powered) enclosure or USB to SATA/IDE-cable
If none of these help, buy another disk, because this one is defective.
Sometimes it shows up in the source list, and sometimes not. If it shows up and I chose it, it will once in a while let me view the contents (movies) and play them, but more often it says that there's no content on the device.
What do I do?