Yes. In most RAID management systems I have seen, the RAID tools allow you to specify which HDD units are to become part of a RAID array when you create that array. Any HDD that is NOT specifically set to be part of an array is NOT a RAID unit, and can be used as a single stand-alone HDD. Of course, if you're installing an empty new HDD, you will need to use Disk Management to Create a New Simple Volume so it can be used.