In a nutshell, if you add an unpartitioned disk to a RAID group with partitioned drives the system will automatically partition it. If you add the disk to a RAID group, or create a new one, with unpartitioned drives, then the system will not partition it.
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I had some odd behavior where ONTAP will mix drives. If you have a FAS2500 with 12 partitioned drives... for adding physical drives, I would create a new raid group then use the slightly larger drives for the new raid group in the same aggregate. Note that if you only have partitioned drives, make sure to leave a virtual spare (9D+2P+1S).. if you have partitioned and physical of the same size, then you can go with only physical spares and use all 12 drives for the data rg0 (10D+2P).