I had multiple Ubuntu VM's installations, and the host is also Ubuntu 10.04 LTS. After I deleted the VM's, the space wasn't reallocated to the host.
Do you know any solution?
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From the VirtualBox Manager you can easily manage you virtual hard disk by selecting File -> Virtual Media Manager or by pressing Ctrl + D.
This will display a list of all virtual hard disks known to the Virtual Box Manager:
Virtual hard disk that are attached to a virtual machine or that contain snapshots need to be released first. Mind that after releasing a hard disk you will not longer be able to run this virtual machine. Deleted machines will automatically be released. Only if a hard disk is not attached to a VM you can remove it:
After a security message you will have the option to free the space occupied on you hard disk by deleting the virtual hard disk file or to keep this file for further usage:
The Virtual Box Media Manager is the preferred method to delete hard disk file as only then all associations to Virtual Box are released securely.
However if you had removed the Virtual Box application you need to manually delete the virtual hard disk *.vdi files that are stored in the location you defined in Preferences -> General -> Default Machine Folder either with your file manager (e.g. Nautilus) or from the command line.