Filer1 is okay. The problem is filer2.

From your attachment I can aggr0 on filer2 is made up of 144GB 15k FC disks.

There are two spare disks on filer2, both are 300GB 10k FC drives.

Because the drives are 10k they won't be substituted with a 144GB 15k drive in case of failure.

However I believe these drives are 15k drives (Hitachi Viper-C 300 GB 15k RPM FC) but the system is only seeing them as 10k drives. You may need to upgrade the disk firmware to NA02 for the controller to recognise them as 15k drives.

Once you update the firmware and the system recognises them as 15k drives the low spare warning will go away.