I have a Thunderbolt drive with two x 3TB drives in it. Until recently, under Mavericks, they were formatted as three x 1TB mirrors. One of the disks failed and I replaced it. I was able to rebuild two of the mirrors, but not the third. I was never able to work out why. I resist upgrading OS X (Have done since it peaked in Mountain Lion) but circumstances drove me to El Capitan. I now need to convert the unmirrored partition and the chunk of space that it never got mirrored to into a striped disk. So, for convenience, I have two disks: Disk A has 2 x 1 TB partitions (5s2 and 5s4) and 1 x 999.6MB partition (5s6). Disk B has 2 x 1 TB partitions (4s2 and 4s4) and 999.6MB of unpartitioned space. I have a simple plan: - Create a new partition 4s6 using the remaining space on disk B. - Unmount the (empty) 5s6 partition - Use diskutil appleRAID to create a striped partition across 4s6 and 5s6. Robert, as they say, becomes my uncle. My plan is to use disk utility to create the missing 4s6 partition. And this is where it all falls apart. The process to do this is not at all obvious. The Disk Futility "partition device" window for disk A shows three partitions that are unreliably described as 1TB each. This is what I expect. For disk B it shows two partitions of 1 TB each. The pie chart does not show any unused space. This is not what I expect. I expect to see a third wedge that shows unallocated space. The "+" button and the Apply button are greyed out. There is no obvious way to allocate a partition for the missing 1TB. Does anyone know how to find it? Thanks for the help, Ploughguy.