Just installed a LSI 9750-8i to my old system with Windows 8.1, added two 2TB disks to SAS-0 port, created RAID1 with them, assigned a letter E: and started cloning data to it. Is it possible to add more disks to LSI's RAID1 without destroying the data on the disks, or I must start over? Documentation is not clear enough about this operation
You cannot "add more disks" to a RAID 1 configuration to extend the available disk space. RAID 1 in a simple sense is a data copy between the disks. You can't add a 3rd disk and maintain a RAID 1.
If you're looking to add a single disk to extend total storage, you should be looking at RAID 5 or 6. But those have a higher minimum disk requirement. It comes down to how many disks you are willing to use and what the end goal of your utilization of your RAID is.