What Happens When You Write Data To Raid While It Is Scrubbing?
Normally, scrubbing runs at low priority and only when the logical drive is idle. Basically, access while scrubbing only slows down the latter. Note that infrequent access can experience higher latency due to the drives being busy during scrubbing.
You should schedule scrubbing for periods when you expect the array to be idle, ie. put it outside your backup window.