How to replace a raid-1 software raid array on Mac OS X with a larger one

1. Offline drive #1.
2. Replace the old drive with the newer, high-capacity drive.
3. zpool replace the drive.
4. Wait for the replacement to be rebuilt.
5. Goto step 1 and repeat for each successive drive in the array.

You may need to export and then import the zpool once this procedure is complete; the pool will then show the increased size. No downtime is required, but if a drive does go south while you're upgrading your array, you will lose everything, as with four total disks you presumably have a 3+1 setup. (The odds aren't that high, but Murphy never sleeps.)

The official source: "Replacing Devices in a Storage Pool" in the ZFS Administration Guide. The ZFS Best Practices Guide is also useful.

That doesn't seem possible in PERC 6/i controllers.

You could break the RAID-1 volume, replace the drive with a bigger one and create a RAID-0 volume on it. Migrate the data, do the software configuration and replace the remaining disk. Not as easy as online expansion though.

You don't mention which software you use but if it is ZFS, you could follow the suggestion above and attach the new RAID-0 volume to the existing disk. Later you destroy the degraded RAID-1 volume, create a RAID-0 on top of the new disk and attach it to the disk. It's very easy.