The last time I messed with RAID was several years ago, so I am not too sure about the current rebuild process WD uses. I looked up some threads on this topic, and it seems the process is somewhat complicated. An expansion needs to be done drive by drive; we cannot simply remove a drive, slot in a new one, and have the unit do the rebuild.
I’ll be concerned if you have already used up the excess space as a spanned volume, in which case, it will need to be deleted, so the likelihood of starting over is high. If you go to Shares and try to create a new folder, do you have the option to choose between Volume_1 and Volume_2? If so, Volume 1 is your 4TB RAID volume, and Volume 2 is the span volume (the leftover space).
If that’s not the case, then I believe the process goes like this: you have to have both original drives (the 6 and 4) in the unit, then go to Change RAID mode, select RAID 1, choose Expand, then it’ll ask you to remove one of the drives before continuing. It’s complicated when done using the WD process. Some users report it works, others say it doesn’t. Have a look at some of the comments and advice here, and maybe get in touch with those who have successfully done this:
If it was me, I’d save myself the trouble and keep aside the 4TB drive (assuming it’s not already wiped, it’s a mirror of your previous RAID volume, so the data should be safe and complete). Start over with the 2x 6TB drives, hook up the 4TB to a PC, and copy over the data manually. But this is not for everyone, and if you have critical data without any other backups, it’s best to look into those threads or ask around in case someone else has a better suggestion.
