You cannot simply copy the dir containing the vdi drive and the other files because every machine must have a unique ID. There is a special command for this purpose

On Linux

VBoxManage clonevdi fileorig.vdi filetarget.vdi

On Windows

<appdir>\VBoxManage clonevdi fileorig.vdi filetarget.vdi

<appdir> is generally c:\program files\VirtualBox.

What if the file is already copied without that command? You can assign a new ID to make it works as well even if you didn't use the clonevdi command.

VBoxManage internalcommands setvdiuuid "filetarget.vdi"