I'm replacing my 2TB parity drive with a 3TB drive.  I'm then going to move that 2TB drive into the array.  Once the 2TB drive is in the array, I'm going to move all the files from an older 400GB drive to it and then remove it from the array.  I'm confused about what will happen the first time I bring the array online after removing the 400GB drive.  Do I have to tell unRAID that the 400GB is no longer part of the array?  Does it have to rebuild parity again?

you can add the 2T as a new drive and copy the contents of the small disk to the 2T and then you have to remove the small disk and build parity again. You can also just replace the small disk with the 2T and then unRAID will start and rebuild the contents of the small disk onto the 2T. If the main screen message doesn't say that the disk has been replaced then you may just have to assign the 2T drive into the same disk slot that the small disk occupied. There will also likely be a check box you have to select asking if you are sure. I would do it this second way. Make sure you do a successful parity check before proceeding.

I'll do it the second way.  I didn't even think about that.  It's building parity right now on the new 3TB drive and I've already precleared the 2TB drive.  Thanks for the advice.