Partition unmounting on WD My Book Studio

I have a WD My Book Studio 1,5 TBB, latest Firmware, not using Smartware (?) Software from WD.  The HDD is connected via Firewire to my iMac (using latest SL)

I partitioned the drive in a 1 TB portion for use with Time Machine (called Time Machine :wink: ) and a 500 Gb portion as Data Storage (called Daten). Lately the Time Machine Partition sporadically unmounts with a window popping up telling me " WD My Book 1112 Media os in use and cannot be powered off. *below and in smaller letters* Try quitting applications and try again". The symbol vanishes from my Desktop.

Using the Disk Manager i can re-activate the partition again by rightclicking the greyed out partition and the Symbol reappears on my Desktop.

How can i stop the partition from unmounting?

If nobody here responds, I’d recommend asking Apple.  I always thought that Time Machine took over the whole drive, not just a partition.