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Disk Utility includes Mac OS X's tools for working with multiple drives so that they appear to the Mac to be a single unit. This is called RAID or Redundant Array of Independent or Inexpensive Disks. The Mac supports two kinds of RAID plus a kind of sort of RAID type. The 2 official RAID types are mirrored RAID, this is a type 1 RAID, and striped RAID, a type 0 RAID. This kind of sort of thing is called a Concatenated Disk Set and it is known informally as JBOD or Just a Bunch Of Disks and no, I am not kidding, that's really what the acronym stands for and they work this way.
A mirrored RAID set writes your data to 2 or more disks, each drive contains the exact same data. If one drive dies or is pulled from the RAID, another will take its place. You would use a mirrored RAID, it is kind of the ultimate backup. For example, if you are running a server from your Mac and you can't afford for it to go down for a second because of the drive giving up the ghost, this is the way to go. Note that the size of the RAID is limited to the size of the smallest volume, so if you have a 500 GB drive and a 300 GB drive, the RAID can only hold 300 gigabytes because obviously the 300 GB member of the set can't mirror more data than it can hold.
A striped RAID splits data among the drives in a set. So for example, half a file may be written to drive A while the other half maybe written to drive B. The advantage of a striped RAID is that it is fast when you are writing and reading really large files, huge databases or video files for example. The danger is that if one drive dies, you have lost access to all your data because it has been split up. A Concatenated Disk Set lets you take your Mac's drives and combine them so that the storage is available in one virtual drive.
For example, you can take 3 to 150 gigabyte hard drives and combine their storage in a concatenated set so that you wind up with the drive that appears to the Mac to be 750 gigabytes. To create a RAID just select drives in the left side of Disk Utility and drag them into the main window. Choose the kind of RAID you want from the RAID Type pop-up menu and click the Create button. In this case, we have chosen a mirrored RAID, it tells us what the storage is because the smallest disk we have here is a 320 gigabyte drive which appears to be 298.1 gigabytes, so that will be the largest that this RAID can go.
We do have a larger hard drive over here, but again it's the smallest hard drive that will determine the size of the RAID. A striped RAID. We have more storage available to us because it's going to split the data up between the 2 drives. And finally a concatenated set is where it combines the storage so we end up with a much larger RAID set. So for example, we will create a mirrored RAID set, and we simply click Create to create the RAID. Note that all the data on the drives will be erased when you create a mirrored or a striped RAID, however, when you create a concatenated set, you don't lose any of your data, the drives just appear to be a much larger drive to the Mac.
And there's our RAID, it appears as a single volume on a Mac and the storage is available to us. And to take a portion of the RAID offline, click the Minus button. If you would like to get rid of that volume altogether, simply click Delete. You will be told that the RAID is going to be deleted, and you end up with 2 separate hard drives again. And just for fun, let's make a concatenated set. Click Create, want to combine those, and here is our concatenated drive on the right and you notice that we have much more storage here because the storage capacity of both drives has been combined.
And we are going to check the RAID, and we are ready to move on to creating disk images in Disk Utility.
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