Deleting the oldest snapshot of virtual machine in VirtualBox is often difficult, sometimes there is error with detail "more than one child hard disk".
Open VirtualBox control panel, select:
File->Virtual Media Manager
click on the Virtual child hard disk within the appropriate virtual machine (don't check for other non-appropriate virtual machines) one by one, at the lower part of the panel, there are some information about the hard disk, "Type", ... "Attached to:", etc., normally all child hard disk of the same virtual machine should be "attached to" the name of the Virtual machine, but at the problem child hard disk, the info. about the "Attached to" is Not Attached, then you can select "Remove" to take it out, if ask whether to "Keep" or "Remove", for safety, you can keep the file, then go back to Snapshots to remove again the snapshot, the error should be gone.
- Similar error occur only on deleting the oldest snapshot but not recent snapshots.
- It is dangerous to just remove the hard disk file stored in the Snapshot directory of the virtual machine, and indeed remove snapshot process of Virtualbox is not just removal of virtual disk file, but some kind of merging the appropriate snapshot content with the current virtual hard disk. Inappropriate removing any snapshot files would be a nightmare to restart the virtual machine.
- In many forums, it is known that the error of deleting the first snapshot is a bug of VirtualBox for quite some years since ver. 3.x, but it still exists till this version 5. A shame to the technology giant, Oracle!