Yes, you absolutely can add the system drive to the pool.   However, we recommend against that for a couple of reasons: Performance. Since the system disk is basically always in use, it may adversely affect the read and write speed on that drive and to the pool in general (read striping should help prevent that for duplicated files). Backup: If you do backup the system, the contents of the pool on this drive can change rapidly and without warning. It may cause your backup program to bloat t