When re-attaching a SR on a fresh VM, I ran into this XenServer tapdisk error. I ran the following from XenServer console to eject all CD drives on that host. I was then able to start the VM with the old image.

xe vm-cd-eject --multiple

If your XenCenter hangs or becomes unresponsive, you can also reset the toolstack from console from a PuTTy session or if XenCenter console is working running the following command:

xe-toolstack-restart

It’s important to note what caused the XenServer tapdisk error. I experienced this on an old version of XenServer 6 when switching out the ISOs from the VMs.