Error -250 : VERR_DEV_IO_ERROR with rawdisk vmdk on MacOSX

[vienin]

I'm trying to use an USB key as a vmdk raw disk on an MacOSX host.

By umounting all mounted partitions of the raw disk device, I'm able to create a raw disk vmdk file properly, but when I try to launch the corresponding VM, VirtualBox raise "-250 : VERR_DEV_IO_ERROR". Partitions of the raw disk are unmounted and not currently in use, but at each time VirtualBox access to device, partitions are automatically mounted by MacOSX, and "-250" error is raised.

So the only solution I have found is to have no partitions recognized by MacOSX on the USB key raw disk (like Ext3), but is really a problem for me.