Issue

You get the following error because you deleted the hard drive outside of VirtualBox Manager and you recreate the same hard drive with the same path:

VBoxManage.exe: error: Cannot register the hard disk '....'{793cc103-f353-4599-aca5-a592a2a885c2} because a hard disk '....' with UUID {81898476-abbe-4a2a-93a9-de3519af209b} already exists

Solution

vboxmanage closemedium disk "/path/to/your/hd.vdi"
# Then, recreate your hard drive. E.g.:
VBoxManage createhd --filename "/path/to/your/hd.vdi" --size 2048 --variant Fixed