Does ONTAP allow mixing of drive sizes in a RAID group?

Yes, ONTAP allows mixing of different drive physical sizes in any RAID group
When adding larger drives to an existing RAID group of smaller drives, the larger drives will be right sized to match the usable capacity of the smaller drives.
For example:
- Adding a 10TB NL-SAS drive to an existing RAID group of 4TB drives results in usable capacity of 3.63TB on the 10TB drive.
  NOTE: Right sizing applies to DATA drives. It is possible for PARITY drives to not be right sized, but additional added DATA drives will continue to be right sized
When adding 10TB or 16TB drives to an existing RAID group of smaller drives, ONTAP does not require RAID-TEC to be used. This is because the usable capacity of the drive is being reduced to less than the drive capacity maximum specified by NetApp Hardware Universe.
To avoid possible performance issues, the minimum number of larger drives that should be added to existing RAID groups are:
- SSDs - minimum of 2 drive
- SAS, NL-SAS - minimum of 6 drives
Provided these guidelines are followed, there is no performance penalty for mixing drive capacities in the same RAID group.

Adding fewer than the recommended drives above might impact the performance of some write-intensive workloads
While changing the maximum RAID size of an aggregate beyond its default size can be used to accommodate more drives in existing RAID groups, doing so might have implications. If creating new RAID groups or adding to existing or resized RAID groups:
- See Considerations for Sizing RAID groups before proceeding with storage aggregate add or storage aggregate modify
Right sized drives can be replaced via storage disk replace if a more appropriate spare is available
If the right sized drive has partitions, follow How to replace a file system partition with a spare partition to move to a partition on a spare that is the correct size