1. Run XE VDI-LIST and determine the UUID of the VM giving you the problem, like this:
xe vdi-list | grep -i -B2 -A22. Once you have the UUID run:
xe vdi-forget uuid=3. Rescan the SR with:
xe sr-scan uuid=4. Now, in XenCenter, go to the VM and click on the Storage tab. You should see it empty. Then click on attach and first entry on the list should be NO NAME. Attach it to the VM, wait about 30 seconds, then power it up!
5. In most cases it should be up and running. If you are still getting errors then wait a minute and try it again. If still not working repeat the previous steps.