I created a virtual disk file VM_1_Ubuntu.vdi. Then I moved it into another folder. I tried to update VM settings (right click on virtual machine -> settings -> Storage -> Controller SATA tab, VM_1_Ubuntu.vdi path). I wanted to set a new path.
It says Cannot register the hard disk already exists
I found a solution
File -> Virtual Media Manager -> Removed existing images (note, I removed them only from the registry).
I followed these steps.
After that I could update the path in the VM settings.