I had the My Book Duo Desktop but It almost fill, I want to buy the new hard drive for it. But my question is: can I take out the old hard drive to connect the PC which should I format it？If need format that will delete all the data that is my worried.
Formatting erases data; there’s no way around it. Depending on the RAID mode you are using (RAID 0 vs. RAID 1) then it may not be possible for your computer to detect one of the internal hard drives, and it would also mean you would not be able to replace/upgrade them.
Additionally, it would be best to confirm if you are referring to the actual new My Book Duo, or if you mean the older WD My Book Live Duo. They behave different, and the WD My Book Live Duo uses a file system that is not compatible with Windows when the hard drives are connected directly.
But my format is independent, I did not use the RAID 0 or RAID 1. And I bought the My Book Duo Desktop not WD Book Live Duo.