Ok, so you have a virtual machine with 1 or x number of hard drives, and you want to copy them into a new virtual machine so you now have 2 parallel systems. You can't manually copy the underlying .vdi file... you must clone them. After you do that, you simply create a new machine and attach the hard drives' copy.
Before you run this, open the original virtual machine and de-attach the hard-drive(s) you want to copy. After you are done copying the file (using the statement below), then re-attach the original hard drive to the original virtual machine.
COMMANDLINE$ /usr/bin/VBoxManage clonevdi '/home/YOURPROFILEUSERNAME/.VirtualBox/HardDisks/ORIGINAL_HARD_DRIVE_NAME.vdi' '/TARGETFOLDER/.VirtualBox/HardDisks/NEW_HARD_DRIVE_NAME.vdi'