My only solution so far was to unplug the Mac from the power outlet and switch it one. After a view seconds I plugged in the Mac and booted it. With this procedure the MyBook was mounted most of the time.
Searching for a better solution I found this alternative. This is not really simpler as the solution I found, but it helps, if in the case my failed.
- unplug the power cord of the MyBook
- unplug the FW800 cable on one side
- keep the Mac running
- plug in the power of the MyBook
- plug in the FW800 cable
No the MyBook will be mounted automatically.