After using any disk with UDF, the disks cease to be unmounted.

A simple test: insert a CD with UDF in the VBox. Update the explorer so that it reads the disk label. Try to unmount CD. VBox gives an error:

Could not unmount the currently mounted media/drive (VERR_PDM_MEDIA_LOCKED).

In debuglog output line like:

(drivers/storage/scsiport/scsiport.c:2894) unknown ioctl code: 0x999204C

If you remove the drive forcibly, this error will always be present when it is discarded, even if the disk is from CDFS.