external hard drive partitioning and RAID striped/mirror combo OR NOT?

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Level 1

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external hard drive partitioning and RAID striped/mirror combo OR NOT?

Intent: to use an 1 TB external hard drive, comprised of two 500 GB drives, to
a) clone a MacBook Pro Intel Duo (~150 GB)
b) clone a PowerBook G4 (~100 GB)
c) serve joint assets, e.g. photos, music, docs, etc. (~500 GB)
d) have spare space for off load or special projects (~250 GB)

After reading 1) Disk Utility Help/ Creating a RAID set*, 2) the WD MY BOOK STUDIO EDITION II
USER MANUAL, and 3) support.wdc.com and search for answer ID# 1787 "How do I make my external hard drive bootable on a Macintosh Computer?"

I used Disk Utility to partition the WD HD into four as above. Then I split Partition c) assets into two 250 GBs to set up a mirrored RAID, but Disk Utility did not allow me to drag the split volumes into the window to make a mirrored array. I guess the "two" have to be physical HDs not just "volumes" to function as mirrors??

Question#1: Continue or Not to Continue?
Question#2: if so, how?
Question#3: if not, should I revert to the original striped format or convert to a mirrored array?

*from Disk Utility Help/ Creating a RAID set:
"You can combine different RAID sets in order to combine their benefits. For example, you can create a RAID set that combines the fast disk access of a striped RAID set and the data protection of a mirrored RAID set. Simply create two RAID sets of one type, then create a RAID set of another type, using the first two RAID sets as the disks. The RAID sets you combine must all be created with Disk Utility in Mac OS X v10.4 or later."

MacBook Pro Intel Duo, Mac OS X (10.4.11), Western Digital MyBook Studio Edition II (1TB)

Posted on Jul 30, 2008 8:29 PM

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Answer 1

Best reply

Kappy

Level 10

361,741 points

Jul 30, 2008 9:31 PM in response to JClayton

Are you trying to create a mirrored RAID using two partitions on the same hard drive? That cannot be done. A mirrored RAID requires two physical drives or two equal sized volumes - one on each physical drive.

Frankly, I don't even see the point of using RAIDs for what you've outlined. You cannot partition a RAID, yet you want two bootable volumes - one for a MBP and one for a PB. The PB's clone must be on a drive partitioned using APT, not GUID as required for the MBP. You cannot do this with a RAID structure.

RAID Basics

For basic definitions and discussion of what a RAID is and the different types of RAIDs see RAIDs. Additional discussions plus advantages and disadvantages of RAIDs and different RAID arrays see:

RAID Tutorial;
RAID Array and Server: Hardware and Service Comparison>.

Hardware or Software RAID?

RAID Hardware Vs RAID Software - What is your best option?

RAID is a method of combining multiple disk drives into a single entity in order to improve the overall performance and reliability of your system. The different options for combining the disks are referred to as RAID levels. There are several different levels of RAID available depending on the needs of your system. One of the options available to you is whether you should use a Hardware RAID solution or a Software RAID solution.

RAID Hardware is always a disk controller to which you can cable up the disk drives. RAID Software is a set of kernel modules coupled together with management utilities that implement RAID in Software and require no additional hardware.

Pros and cons

Software RAID is more flexible than Hardware RAID. Software RAID is also considerably less expensive. On the other hand, a Software RAID system requires more CPU cycles and power to run well than a comparable Hardware RAID System. Also, because Software RAID operates on a partition by partition basis where a number of individual disk partitions are grouped together as opposed to Hardware RAID systems which generally group together entire disk drives, Software RAID tends be slightly more complicated to run. This is because it has more available configurations and options. An added benefit to the slightly more expensive Hardware RAID solution is that many Hardware RAID systems incorporate features that are specialized for optimizing the performance of your system.

For more detailed information on the differences between Software RAID and Hardware RAID you may want to read: Hardware RAID vs. Software RAID: Which Implementation is Best for my Application?

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Answer 2

JClayton Author

Level 1

5 points

Jul 30, 2008 10:06 PM in response to Kappy

Thank you. I understand what you are saying in general, and I think it is what I had gathered, that it has to be two physical HDs not two volumes. The way the Disk Utility help read, made it sound as if Disk Utility could be used to create a virtual solution. It seemed as if the instructions* at support.wdc.com for answer ID# 1787 made this sound like a possibility. I will gladly read the tutorials.

As to using RAID or not to use RAID may not really be an option. The WD MyBook Studio Edition II comes as a striped array across the two 500GB drives A&B. So I still have the question as to whether to partition it as described a)-d) or just treat it as one big volume that I stick folders into!

I can hold on until tomorrow to decide.

Good night,
J.

*The external hard drive must contain a HFS+ GUID or APM (Apple Partition Map) partition in order to boot.For directions on how to partition and format an external hard drive using the HFS+ GUID or APM partitioning scheme, please follow the directions below:

1. Ensure that the external hard drive is connected to the computer and powered on. Open the main drive (the drive that contains the operating system).
2. Open the Applications folder.
3. Open the Utilities folder.
4. Select Disk Utility.
5. The drives are displayed on the left side of the window. Click on the drive that you wish to partition (the top drive listing for the hard drive that you want to work with).
6. Once the drive is selected click on Partition.
7. Click on the Options button to select the partitioning scheme.
8. Click on GUID or APM and then click on the OK button.
9. You will see several options for setting up the drive. Once you have selected the number of partitions, the format type you want (HFS+), and a volume label, click the Partition button.
10. Following the completion of this process, the drive will appear on the desktop.

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Answer 3

Kappy

Level 10

361,741 points

Jul 30, 2008 10:53 PM in response to JClayton

The problem I see, then, is that if these two drives are already configured as a RAID 0 (striped) via a controller in the box, then there's little that you can do. Once configured as a RAID you cannot then partition the RAID. If you cannot have two separate drives then you cannot pre-partition the drives before setting up RAID arrays using individual volumes from each drive.

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Answer 4

JClayton Author

Level 1

5 points

Jul 31, 2008 4:51 AM in response to Kappy

Thanks again.

Actually, even though shipped as striped, the WD drive comes with software that allows reconfiguring to a mirrored array.

So maybe I'm looking to do something too complicated, but in an ideal world, I could partition half of A and half of B, mirror those as a pair and have half of each of the two drives remaining to use for straight backup, unmirrored, unstriped.

Crazy, huh?

I imagine you're thinking why don't I just buy another ExHD? Well, this was a stretch of my $$$ but still cheaper than buying a new computer to replace the PB G4.

Obviously, I'm trying to do a bunch of stuff with limited resources, but hey, at least there's no fault for lack of imagination or lack of trying!

J.

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Answer 5

JClayton Author

Level 1

5 points

Jul 31, 2008 9:57 AM in response to JClayton

Conclusion:
After reading everything I could find, including the references Kappy provided, I decided to try the following:

1) I used the WD Drive Manager, software that came with the WD MyBook Studio Edition II external hard drive, to reconfigure it from striped (RAID 0) to mirrored (RAID 1).

2) then I used Tiger's Disk Utility to partition the resulting "half-sized" volume into three, for uses a), b), and c) above.

Upside:
• addressed my most pressing issues to have duplicates of data from and freeing up space on each of our computers, plus a volume for joint assets;
• peace of mind redundancy, without having to manually copying files to two separate HDs.

Downside:
• no spare space, since half was used for the mirror;
• if I set up a bootable volume, it can only be for one computer, not both, since they are different generation Macs;

As I write this, I am copying data from our computers. So far so good.

I will leave this open for a few days, in case someone has another idea to share.

J.

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