want to update to LR6 but most of my images are on external HD....shall i disconnect and then will it upgrade when reconnected?
What gets upgraded is the catalog. So yes, you need that hooked up if on an external drive. After you upgrade, the original older catalog will remain as a backup, a new catalog for LR6 will be built. I'd keep the older version around awhile to be safe. You can't go back from an LR6 catalog to LR5, so keep that in mind.