Your question is not clear, I think you’re asking if you start with two drives in RAID 1, add two new drives, then change RAID mode to RAID 10, can you do this without data loss? The answer is NO. All four drives will be formatted as RAID 10, thus, all data is erased from all four drives.
RAID 1 is made up of two drives only, while RAID 10 is made up of four drives. All the drives should be the same size for optimal performance and date capacity.
If you currently have two 10 TB drives in RAID 1 you will have a total of 10 TB useable data in one volume. For example, you could call it data 1 and map it as drive P:
If you add two more 10 TB drives you can create a second RAID 1 volume with 10 TB of useable data. All the data on your existing volume will stay intact. For example, you could call it data 2 and map it as drive Q:
The grand total would be 20 TB of useable data across two volumes = two mapped drives P: and Q: P: would have all your existing data and Q: would be anew empty drive.
If you want to all 20 TB in one volume you have to migrate to a 4 drive RAID 10 with data loss
you could migrate to RAID 5 and have a total of 30 TB in one volume / one mapped network drive without data loss!
Refer to answer 20862…
In any case, I would always backup my existing data to an external USB drive first. If it is critical data, I would backup to two different external USB drives before making any changes.