I don't know of any way to format the disk from the host. You need to boot a VM and format the new disk within a guest. If you create a new disk be aware it is unformatted and cannot be mounted. When you've created a new fixed format disk image you need to go into the settings for each VM that uses the dynamic disk and replace it with the newly created fixed disk. VBoxManage clonemedium disk old.vdi new.vdi -variant Fixed Use this command to convert a dynamic file into a new fixed file. You cannot mount dynamic disks, they must be fixed. Mounting fixed size disks only (no Dynamic disks) All of these things are done on the host unless otherwise stated.
I'll try to update this if I find new information as I use this. I tested this all running VirtualBox 5.0.2, Mac OS X 10.9 as the host, and Mac OS X 10.6 as the guest.
#Original mac os x 10.0 virtualbox how to
Read on if you want to know how to mount the filesystem and how to make that filesystem bootable. I also wanted to be able to mount the guest filesystem on the host. I tried cloning existing images and resizing them but when I booted the enlarged images Disk Utility would never let me expand the size of the volume. I wanted to be able to create OS X VirtualBox images quickly and easily without having to run an OS X installer every time.
Maybe someone else will figure out how to do this. The reason I am posting it is because it's a lot of interesting info. I never posted this because after using VM's I created this way for a few days they quit booting. I wrote this back in Sept 2015 but am just posting it in Apr 2016. I added more than 3 times the information and added a whole new page discussing the UEFI interactive shell. DIY Capacitive Stylus for iPad and iPhone.Mounting VirtualBox HFS+ VDI files on OS X.Reading the keychain with a perl script.Mounting VirtualBox HFS+ VDI files on OS X - Magnusviri