Like many other users of virtual box the I had the problem that the original size of my virtual hard drive was too small. Unfortunately the the documentation of virtual box does not really help in this case. Fortuantely there are a lot of users who have proposed ways how to do it.
My favourite ist definitely tu use gparted for cloning and resizing since it’s straightforward. A very good description can be found
here
Though the screendumps are no more up-to-date, the description is still valid.
Remarks

• First I tried to use an existing virtual drive as the new primary master. This did not work!
  The virtual drive must be created newly.
• Before you can use the new hard drive gparted must generated a partition table for it. It will ask for it if it does not exist.

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