A possible alternative is just to /mir both HDDs.
Don't do that. You don't want to touch OS and application folders that are all inter-dependent and have corresponding registry entries. Focus on specific data folders like c:\Users\MyAccount\Documents.
Use the /l switch to test with. It will tell robocopy to list the files/folders that it would copy without actually moving any data. Pick one folder to start with. Do a test run with /l and verify that it's going to copy the files that you want/expect.
For each data folder you need to decide if the source or the destination contains the most current data. /Mir will overwrite/delete any file on the destination that does not match the source. If you have updated files on the destination folder and want to keep them then use /xo.