Hi David, & Welcome to Tom's Hardware!
Tell us as much as you can about your computer setup, manufacturer, MB, CPU, Win version, make, model #, size of your HDDs, etc. Also how you have your RAID 1 installed (as a Storage Space Mirror in Win-8, as RAID 2 in Win-7, in a NAS storage unit as a network component). Also other HDD's on your system and how they are configured.
That way someone will be able to give you the best answer to solve this issue.
As a general rule with RAID 1, you can remove it and replace it with the same size HDD, and after rebooting, resynchronize the new drive in the Mirror.