Old Data

Witht the upgrade to Catalina, I now have what acts like 2 partitions. one is the new Catalina partition and the oplther is the Olde Mojave patrition. The old "Partition" is accessed as "Macintosh HD - Data" while the new is just "Macintosh HD" The old one is acting as "read only" any suggestions on how to merge or fix? In disk utility the old ones space is grayed out as what looks like unaccessible