Hi
My case is nearly similar, the difference is: my MBL is my backup.
I have three 2.5’ WD passports and I regularly connect them to my laptop and backup them to my MBL using the free SyncBack software.
If you do not have a gigabit router, you will be limited to around 10 MB/s
If you have a gigabit router, then you will probably be limited by the MBL HDD write speed (around 30 MB/s)
Hope that helps
