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VBoxManage sharedfolder

VBoxManage sharedfolder allows to add or remove shared folders from/to a virtual machine: 
VBoxManage sharedfolder add <uuid | vmname> <--name=name> <--hostpath=hostpath> [--readonly] [--transient] [--automount] [--auto-mount-point=path]

VBoxManage sharedfolder remove <uuid | vmname> <--name=name> [--transient]

Default mount location when not specifying an auto-mount-path

j When specifying auto-mount without an auto-mount-path, VirtualBox will choose the location where the shared folder will be mounted.
For a Windows guest, VirtualBox chooses a drive letter (for example Z:).
For a Linux guest, the shared folder is (probably) mounted at /media/sf_$SHARENAME. The exact location can be determined like so: 
$ mount | grep vboxsf

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