If the hardware provider veto's the resync operation you may end up with the target LUN offline. If the operation is to resync to a target LUN that is not the same as the original source LUN the target may end up offline after the veto. The veto here is the correct option for the provider as it should fail to overwrite this volume. From Himanshu's blog: LUN-RESYNC WITH DISKSHADOW AND VIRTUAL STORAGE
NOVOLCHECK: This diskshadow flag corresponds to VSS_RECOVERY_NO_VOLUME_CHECK of VSS_RECOVERY_OPTIONS during RecoverSet and helps protect one or more unselected volumes on the target LUN from being silently overwritten during a resync operation. Specifying this option would indicate that the operation need not do the above check and it is OK to overwrite all the drives on the target LUN. A VSS resync operation as in the above scenario will fail in absence of this flag.
NOVOLCHECK: This diskshadow flag corresponds to VSS_RECOVERY_NO_VOLUME_CHECK of VSS_RECOVERY_OPTIONS during RecoverSet and helps protect one or more unselected volumes on the target LUN from being silently overwritten during a resync operation. Specifying this option would indicate that the operation need not do the above check and it is OK to overwrite all the drives on the target LUN. A VSS resync operation as in the above scenario will fail in absence of this flag.