Restrictions apply for Global Mirror when upgrading from V7.1.0 or earlier to V7.2.0.
Resolving the problem
Before upgrading a system to V7.2.0 any Global Mirror relationships that have their secondary volume on the system that is being upgraded must be stopped. These relationships can be restarted after the upgrade has completed.
This restriction does not affect Metro Mirror or Global Mirror with Change Volume relationships (cycling mode set to multi), which can continue to operate while the upgrade is in progress.
The software upgrade test utility can be used to check that Global Mirror relationships have been stopped before starting a software upgrade to V7.2.0.
See more details here