I do not use rsync with local drives but Rsync is wonderful for sync, cloning, backup and restore of data between networked linux systems. A fantastic Linux network enabled tool worth spending time learning. Learn how to use rsync with hard links (--link-dest=) and life will be good.
I, however, have found rsync not as useful with local drives.
In the name of speed, Rsync, in my experience, changes automatically many operational parameters when rsync believes it has detected two local drives. Making matters worse. What is considered "local" from rsync's perspective can at times not really be that local. One example rsync sees a mounted SMB network share as a local drive. One can argue and be correct in explaining that in this case for rsync as a program instance the drives are all "local" but this misses the point.
The point is that scripts that operate as expected when used between a local and a remote drives do not work as expected when the same script is used where rsnyc sees the data paths as two local drives. Many rsync options seem changed or do not seem to work as expected when working with all local drives. File updates can slow to a crawl when one of the "local" drives is a networked SMB share or a large slower USB drive.
For example with "cwrsync -av /local/files/ /mountedSMBshare/files" and no -c (checksum) option where need for transfer should to be determined by file size and date with all local drives I see whole files copied between source and destination when the files have not even changed. This is not helpful behavior when one "drive" is a slower SMB networked share and the other a slow NTFS USB drive. A ssh into the SMB share server would be much better but this is not always possible and windows stuff, much hated, is part of everyday commercial life.
I would have preferred that rsync's operation was consistent regardless of the drives "location" and simply provide a --option for the user to invoke "local" operation when a speed advantage was seen as available and helpful. In my humble opinion this would be more consistent operation making rsync easier to use and more functional.