Use the Disk Utility in the /Applications/Utilities/ folder or a program such as
SuperDuper or
Carbon Copy Cloner to duplicate that drive to another drive or partition.
Use the free Carbon Copy Cloner. After the first clone use the incremental setting and delete anything on the destination not on the source setting and it will take only minutes after the first time.
Disk Utility => Restore is handy for mounting and copying full volumes between drives, for restoring from a sparse disk image, when you don't have an alternate boot drive for CCC and SuperDuper.
CCC/SD do smart copy updates, DU Restore just copies new files.
They all have their uses and place and worth learning.