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Looks like the Mac OS 10.5 (Leopard) Disk Utility has even more issues than the partition resize bug that I already talked about.

Fundamentally it seems to fail when trying to add members to a mirrored RAID Volume. I tried several ways to do this but every time I try I get one of these fundamentally useless error messages: “Unrecognized Filesystem”.

Turns out you need to go back to the command line and use the underlying tools that the GUI tool is supposed to use. Here’s a description of the steps that worked for me:

• diskutil list
  Now find the Identifier of the partition or disk you want to add; it’s something like disk4 for a disk or disk4s2 for a partition. While you are at it, make sure that the disk that you want to add has the same partition scheme as the one that holds the existing RAID slide - otherwise this won’t work, anyway.
• sudo diskutil addToRAID member Identifier /Volumes/Mountpoint
  where Mountpoint is the name under which the original RAID is mounted (TAB-expansion will give you the options of what’s there under /Volumes/, so this should be rather straight forward). Note that this command should be issued as superuser, so you need the sudo
• diskutil listRAID
  will show you the progress of rebuilding the mirror. Depending on size and connection speed this can take a long time. I did a 500GB RAID of two external disks connected via USB and that’s estimated to take 16 hours…

I wish they paid more attention to the functionality of their tools. Being pretty and easy to use is nice - but it actually needs to work, too.

Thanks for visiting!
I hope this was helpful - if not, please leave a comment and let me know why! Were you searching for something else? Did I miss an important aspect?