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I have been struggling with trying to get Disk Utility to allow me to divide a 4TB HD into two 2TB partitions, and then have a mirrored raid of that for redundancy. Is this not possible?

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  • Have you looked at: Create a disk set using Disk Utility on Mac – user3439894 Oct 18 '20 at 3:41
  • Did you think about creating a RAID from both disks and creating the two partitions on the RAID instead of RAIDing the partitions? – Ant Oct 18 '20 at 8:37
  • What @Ant said. you create the RAID FIRST and then partition it. So you could easily have two 2TB partitions on your RAID-1 array. But you can't RAID only part of a drive. RAID = Redundant Array of Independent Disks. – Steve Chambers Oct 18 '20 at 13:27
  • @Ant I did try this, and it wasn't working.. If I recall as soon as I did the partition it killed the raid slice. Even Disk Utility says "Changing a raid partition may leave the raid set unusable", which was what happened for me. – patrick Oct 19 '20 at 1:35
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It is not possible and even if it were it would be a very bad idea.

RAID 1 is to protect from hardware failure, and as described in the Apple document Create a disk set using Disk Utility on Mac mentioned by @user3439894 in comments, requires multiple disks (not partitions).

  • Mirrored (RAID 1) set: Protect your data against hardware failure with a mirrored RAID set. When you create a mirrored RAID set, your data is written to multiple disks so the information is stored redundantly.

Even if it were possible it would provide no protection against hardware failure (as both partitions are on the same disk) and in the case of a HDD would drastically reduce write performance as the disk arm would have to continuously jump back and forward between partitions.

While it may theoretically give some limited protection against bad blocks if you only have one disk you would be much better off making a a backup to another partition (which would protect you from accidental file deletion). Backup to another to another disk would protect you from disk failure and file loss while RAID 1 is only designed to protect from hardware failure.

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  • I don't think you understood my question. I am not looking to have a redundancy raid across two partitions-- I am looking have a drive split into two partitions, and another drive be the redundancy backup for those two partitions. – patrick Oct 19 '20 at 6:17
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Disk Utility's RAID support is patchy at best.

I recommend using SoftRaid. The 'Lite' version is sufficient for basic needs, and costs $50.

https://www.softraid.com

I use it on my G-RAID unit. I've replaced the drives once, and SoftRaid handled the re-population of each new drive in turn effortlessly.

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