HP Storage Mirroring V5 Software - Manual (Page 26)

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NOTE: When performing a rolling upgrade, update the target servers first. After the upgrade is complete and the
target servers are rebooted, the sources will automatically reconnect to the targets. Upgrade the sources
when convenient.
If you are using a chained configuration, update the last target first, then update the middle server acting
as both a source and target, and update the production source last.
If you are using a configuration where the source is an older version than the target, you will not be able
to restore from the newer version target back to the older version source. You must upgrade the source
to the same version as the target before restoring.