Recently, I had a situation where my the VDI files for my Windows 10 virtual machine expanded in size all of a sudden and I needed to reduce them. My downloading maps for use with Routebuddy may have been the cause so I moved the ISO installation files onto the underlying Linux Mint drives. With that space, I then set to uncovering how to compact the virtual disk file and the Sysinternals sdelete tool was recommend for clearing unused space. After downloading, I set it to work in a Powershell session running on the guest operating system from its directory using the following command:
./sdelete -z [drive letter designation; E: is an example]
From the command prompt, the following should do:
sdelete -z [drive letter designation; E: is an example]
Once, that had completed, I shut down the VM and executed a command like the following from a bash terminal session:
vboxmanage modifyhd [file location/file name].vdi --compact
Where there was space to release, VDI files were reduced in size to return more disk space. More could be done so I will look into the Windows 10 drives to see what else needs to be moved out of them.