Changing the gid and perms on a device

[JD]

I have a flash disk which is detected as /dev/sdc

I need to use it as a virtualbox device.
So I created a vmdk for the disk and registered it.
Furthermore, virtualbox requires that in order to use
the device, the device must belong to the group
vboxusers.
So I chmod'ed it to vboxusers.

But somehow magically, it reverts back to being
owned by the group disks.

I have disabled selinux. So I have no idea what/who
is changing it back. This is the reason why I am asking
this list. The virtualbox list does not seem to have
an answer.