I've got a Mac Pro 1.1 running macOS Server 10.6.8 that I want to use as a RAID-Server. I use Disk Utility to run a RAID1 set with two 500GB hard drives.

Now adding a third 500GB hard drive to the existing RAID-Set with diskutil appleraid add member fails by showing me the following error message:

Error: -9911: The target disk appears to be too small to add to this RAID set

But when I delete the RAID-Set and set it up with all three hard drives together, the RAID is being created without any problem.

I belive that it could have something to do with the fact, that the third hard drive is a different model (different company), and the other two are the exact same. But if that would be the case, then I could not replace a hard drive in a RAID-Set, even if it would be from the same company and the same model line – since even those can change over time – except if I buy the exact same one – right?

I want to be sure that Apple RAID is capable of adding a new drive to an existing RAID if I have to swap a damaged one. Is anyone able to help me with this problem?

Thanks in advance!

  • RAID-1 is TWO disks. It is called a "mirror" meaning disk one is a mirror (duplicate) of disk 2 you cant use 3 disks in RAID-1. You can do a RAID-5 array though with 3 disks. Commented Aug 28, 2019 at 12:53
  • As far as I know, you can mirror onto as many drives as you want, given the controller supports it (hardware RAID). But I am using a software RAID, which should give me a RAID1 with as many hard drives as I want. Check this out: superuser.com/questions/489793/…
    – Alexes
    Commented Aug 28, 2019 at 13:27
  • Don't cross post to multiple stack exchange sites. Pick one & stick to it - superuser.com/questions/1476652/…
    – Tetsujin
    Commented Aug 29, 2019 at 10:56
  • @Tetsujin sorry for that – didn't know about that rule.
    – Alexes
    Commented Aug 29, 2019 at 11:34

1 Answer 1


I did not know, that it's not ok to cross post over multiple platforms – sorry for that.

However on superuser I got an answer to my problem, so here is the link once again: https://superuser.com/questions/1476652/adding-a-new-hard-drive-to-raid1-software-raid-fails

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