I'm running a Linux Mint guest OS inside a Linux Mint host OS in Virtualbox, and I've noticed that dragging an icon from the guest OS desktop onto the host OS desktop simply pushes the icon to the edge of the desktop, instead of dropping it onto the other desktop. On one occasion (by sheer luck), I successfully dropped a file from a Linux Mint guest onto a Linux Mint host desktop, but on all other other occasions, it didn't work (and I got the same error message.) I have installed guest additions, and I'm also accessing the .vdi image from a separate hard disk partition (the .vdi is on a separate Windows partition, on the same computer.) What are some possible reasons for this problem?
I'm using VirtualBox with KxStudio (which uses Ubuntu). It gets stuck at 99% here too. But if you click on the x to close the window, and then go to Home> Documents > VirtualBox Dropped Files, you find it there in a subfolder.
You can then copy it to its desired destination on the guest. So still useful as a way to get files to the guest from the host OS. Might be a useful tip for others in same situation?