For what it's worth, I managed to find a solution to my problem. It was a problem in Windows that somehow the automount of drives was set to disabled.
Trying to find a solution for a non-DrivePool related problem I manually executed a command to enable the automount of drives.
As a test I removed the driveletters from my drives with solely Pooldata on them. After a reboot the drives still show up in DrivePool.
What I did:
1) open an elevated Command Prompt (cmd)
2) type diskpart
3) type automount enable
4) reboot
