MyBook Thunderbolt Duo - Upgrade drives?

[ERmorel]

Hello and welcome to the WD Community.

Not sure if that’s might be possible, although, some users were able to do that on a different unit.

See if the following link helps

http://community.wd.com/t5/External-Drives-for-Mac/Upsizing-a-1-TB-My-Book-Studio-Edition-II-in-RAID-1-to-2-TB/m-p/8245