VirtualBox VERR_NOT_SUPPORTED opening Windows 10 partition/vmdk
This was (or obviously still is in VirtualBox 5?) a known bug of VirtualBox 3 which should have been fixed in 4.0.0!

The workaround is disabling the use of the new async I/O code... done by ticking "Use host I/O cache" for the SATA (and SCSI) controllers.