virtualbox.org

I dualboot Mandriva and Ubuntu, and have created several VMs under Ubuntu. Under Mandriva, all the virtual machines and hard disks are listed as inaccessible.
I can fix this by unregistering and reregistering one virtual hard disk under Mandriva, and then create a new machine which uses this disk. It takes me perhaps two minutes to do this. While the new VM boots up, the whole list of VMs suddenly changes from inaccessible to OK.
But when I boot the host as Ubuntu again, all the VMs are inaccessible, and I have to repeat the process to get the list OK.
(This is under version 3.0.6. The changelist for 3.0.8 doesn't seem to address this problem.)
This is very annoying, as I expect the virtual machines to be portable. It merely seems to be a matter of which host system shall own the directory containing the virtual hard disks, but why should one host own this directory?
VirtualBox should really be fixed to automatically rediscover/repossess that directory!

I can't really say about native Linux file systems, but I share my VMs between Ubuntu on my PC and Laptop through my external hard drive, which is formatted as NTFS. With that, I have no problems. I have them both look at one particular folder, config files and all, without issues. Maybe you need to move them to a more generic location on your system, including the XML config files. You might need to edit the files manually to reflect the new location.

I have the VMs in a separate ext3 partition, and the host systems in other partitions on the same internal hard disk. (So that they can repair each other) This should really give enough separation.

It should, yes, but are you sure that the group and user ID's are the same on both systems when it comes to the ext3 privileges?