I setup a Debian guest on a Debian host. I installed so many applications, I rebooted so many times, I took a few snapshots. Now I needed to do something with VirtualBox settings for that machine, so I shut it down. After making the settings, I wanted to start machine, but it didn't boot. It says "no bootable media". Of course, because no harddisks seem to be attached to the virtual machine. So I (re)attached the vdi image to the Sata slot. I booted, but the machine was in the original state. Snapshots could be seen in the Snapshots tab, but they are simply ignored.
How can I fix this?
Edit
I rebooted the pc, removed all my Virtual Machines, I even reinstalled VirtualBox. Here is the latest error:
Failed to open virtual machine located in /home/....-1.vbox.
Medium '/home/....-1-disk1.vdi' cannot be closed because it is still attached to 1 virtual machines.
Result Code: VBOX_E_OBJECT_IN_USE (0x80BB000C)
Component: Medium
Interface: IMedium {05f2bbb6-a3a6-4fb9-9b49-6d0dda7142ac}
Callee: IVirtualBox {fafa4e17-1ee2-4905-a10e-fe7c18bf5554}
Callee RC: NS_ERROR_FAILURE (0x80004005)