Where you install the external storage system depends on the number of sites in your configuration. In a three-site configuration, you install the external storage system in a third site away from the primary and secondary sites. I/O from servers continues if any failure occurs at the primary site, the secondary site, or the site where the external storage system is installed.
In a two-site configuration, you install the external storage system at the primary site. If failure occurs at the secondary site, I/O from servers will continue. However, if a failure occurs at the primary site, I/O from servers will stop.
At the secondary site, you cannot install any external storage system for quorum disks.