Machine-based HotAdd will fail over to Machine-based LAN with error "3201 - can't open" [<datastore>] <VM name>/<VM name>.vmdk if ESX host does not have proper licensing.
The ESX host is not properly licensed, which does not include Hot-Pluggable virtual HW Product Feature.
To check your licensing level and associated product features:
Using the vSphere Client to select the host on which the VM in question is located.
In vCenter 4:
In vCenter 5: