Partitioning a hard drive is like dividing a hard drive up into several sections. You may use the partitions for reasons such as data storage or as a startup drive.
You can partition a hard drive even if there is data on the drive. You just have to know how to do it correctly to keep your data intact.
If you desire to partition your hard drive, or even re-partiton a hard drive, in the below video are the steps to do so, using Disk Utility in Mac OS X El Capitan.
As always, before you do any of this, make sure that your drive is backed up. You don’t want to lose any data.