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!


Hard disks in exact sizes:

Drive 1&2 (They are the same models): 500.1 GB (500107862016 Bytes) (exactly 976773168 512-Byte-Blocks)

Drive 3: 500.1 GB (500107862016 Bytes) (exactly 976773168 512-Byte-Blocks)

  • Can you tell us the exact sizes in bytes (edit the question). The third disk may be a little smaller than the other two. I guess when you set it up anew, the RAID adjusts to the smallest device, so it's not a problem. I don't know diskutil appleraid enough to help you further, therefore just a comment. – Kamil Maciorowski Aug 29 at 10:25
  • Don't cross post to multiple stack exchange sites. Pick one & stick to it - apple.stackexchange.com/questions/367756/… – Tetsujin Aug 29 at 10:56
  • @KamilMaciorowski Your comment was just the right one! I checked the exact size, and the third is indeed slightly smaller. I've put it in the edit. Thank you! – Alexes Aug 29 at 11:21
  • Creating partitions/filesystems inside the RAID array, so they don't use the final fragment of each disk, should allow the array to be rebuilt on smaller disks. This assumes diskutil doesn't destroy data while creating a new array. Theoretical note. Totally untested. – Kamil Maciorowski Aug 29 at 12:09

There is a small difference in size from vendor to vendor. They basically usually add few KB for things like sector reallocation.

It is normal if your initial disks had a few more bytes not to allow a smaller drive to be added, because those few extra bytes will be unable to be mirrored.

So make sure to either use identical models of drives or larger ones. The exact size is for some vendors specified on the drive. If not, you can certainly determine it in multiple ways.

  • I did know that hard drives always differ slightly in sizes – but I never knew of the fact, that distinct models would have the exact same size, and therefore falsely assumed that a RAID1 setup would be tolerant in some way. Thanks you very much! – Alexes Aug 29 at 11:26
  • They all keep the same exact size for a specific HDD series. But even so, even having an identical backup model on stand-by, it's a good practice to leave a little unallocated space. For example, the 500GB disks have 465.8 true GB. The value above 465 I leave unallocated (in this case it's .8GB - quite a lot but enough for many situations). A similar thing can be done for any drive size. – Overmind Aug 30 at 6:35

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