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I have set up a RAID5 device with three 2TB hard drives using mdadm. The device was successfully created, but I cannot seem to create a partition on the device.

When I try to make an ext3 or ext4 partition via Disk Utility, I get the following error

Error creating partition: helper exited with exit code 1: In part_add_partition: device_file=/dev/md0, start=0, size=4000526106624, type=
Entering MS-DOS parser (offset=0, size=4000526106624)
found partition type 0xee => protective MBR for GPT
Exiting MS-DOS parser
Entering EFI GPT parser
GPT magic found
Leaving EFI GPT parser
EFI GPT partition table detected
containing partition table scheme = 3
got it
got disk
new partition
guid '' is not valid
type '' for GPT appear to be malformed

I have seen this question, but that seems to suggest using gparted to do the partitioning. I'm fine with doing that, but my RAID device doesn't show up in the list of gparted devices. I suspect because this is a RAID and not a regular disk.

I have already created a GPT partition table on the device. How can I add a partition to my device?

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Check gparted again; it does show raid devices. – psusi Nov 10 '12 at 23:30
@psusi Here's what I see in Gparted.. You can see the device in Disk Utility on the bottom listed as /dev/md0. I don't see it above. – Kris Harper Nov 11 '12 at 3:00
up vote 3 down vote accepted

My fault. I didn't realize I couldn't create a partition while the array was rebuilding.

I thought I had done this in the past, but I guess I waited first.

Once I waited for the rebuild to finish, I was able to successfully create a partition.

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