Recently, I added a new hard disk drive to my PC and moved the partition where I have my virtual machines data and disks to a spacier partition in this disk. Later, when I opened VirtualBox, it complained about not findind the virtual disk of my virtual machine, and opens the disk manager, but there there is no option to relocate the disk, just a remove option which MUST NOT be used. Just close the disk manager, head to Preferences and update the paths to the default folders. Restart VirtualBox and everything should be right on track again.
The point here is disk manager has no option to relocate disks because all virtual disks are expected to share a single path, not scattered in various locations.