If you are receiving the following error in XenServer/XenCenter, there is a quick fix!

4/05/2011 4:05:40 PM Error: Migrating VM ’36′ from ‘blXXX’ to ‘blXXX’ – Internal error: INTERNAL_ERROR: [ Failure("The VDI 996b046b-1960-4315-bad7-48830254d4c3 is already attached in RO mode; it can't be attached in RW mode!") ]

Or

5/05/2011 7:09:08 AM Error: Migrating VM ‘n3 – NTP’ from ‘blXXX’ to ‘blXXX’ – Internal error: INTERNAL_ERROR: [ Xb.Invalid ]

This can be fixed by running:

xe-toolstart-restart