replacing bad drive with a different size

[dbconz]

model: LS420D
firmware: Version 1.85-0.01

one of the WD Green 4TB drives failed on my RAID1 array.

since this device is 3 years old, i choose to buy two new 6tb drives.
i wanted to replace the bad one (bay 2), let the array rebuild, then replace the other drive (bay 1).

when i put the new 6tb drive in bay 2, it's not giving me the option to rebuild the array.

can someone point me in the right direction?

[dbconz]

looks like i sorted it out.

the new drives had some sketchy reviews on NewEgg, and one person said they ran a health check on the new drive and it had a few thousand hours on it.  so when i ran a health check on my new drive, i must've initialized it on the Disk Management.

once i attached the new drive back to my laptop and ran the CLEAN utility to remove the partitions, the LS420 was able to start rebuilding the array