virtualbox.org

I would like to use one virtual Debian-installation on an USB-drive and want to work with 2 Computers (Windows/Linux). On each of them the data structure is equal. The guest additions are working fine... but:

When I'm opening the virtual Debian from Windows - the shared folders are working. When I'm opening the same virtual Debian from Linux the shared folders have write only permissions. Even the checkbox to give write access is missing under linux.

Is there a reason?

I found one possibility: It is needet to unmout the Debian-Guest-HDD via Virtualbox-GUI bevore starting virtual Debian in Linux. The other way (first Linux then Windows) - works without unmounting.

Is it possible to set an option that this unmounting of hdd works automated?