My Book Mirror - do files copied directly to it mirror or does WD Anywhere Backup software required?

I have a My Book Mirror 2TB set to RAID 1 configuration. The laptop I use the My Book drive with is close to capacity (10GB free of 150GB)

I have a photos folder that is 25GB (these are images only take in the last year and expect this potential double every year)  that ideally should move off my laptops hard drive to free up disc space.

I know that these could be copied directly to My Book but will this mirror the data or does mirroring only happen with using WD Anywhere Backup software? I’ve used only 12% of space on My Book Drive so the capacity is there.  Or is my only option is to get another external hard drive and include this in the backup plan within the WD Anywhere Backup software?