When you create a VirtualBox virtual-machine, you get the option to choose either a fixed sized hard drive (in which case the entire amount of space is allocated and fixed immediately), or a dynamically expanding hard drive for the virtual machines OS and file storage (i.e. you specify a maximum size for the hard drive, it starts at 0 bytes, and increases as necessary up to the maximum you gave it on creation - at which point the drive is full). This is all well and good, but the problem with dynamic storage is that although it's more than happy to increase in size, it doesn't come down in size again when data is removed. So, to give an example, if you created a 10GB dynamic disk for a virtual machine it starts off at 0 bytes, you install a 1GB operating system and the drive is now 1GB in size (and hence taking up 1GB of space on your actual hard drive), you rip a 4.7GB DVD to the virtual drive which makes its size now 5.7GB, you delete the DVD rip so only the OS remains - and you might think that the
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