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[michaelnotar]

i have two HHDs in my computer, #1 has OS and personal photos, #2 has work images.

can one drive be set up to mirror itself? if so, is it any safer?

i'm considering external HHD, one that does RAID, to backup HHD #2 and have a question.

which senario is better... 2 external HHDs, 500GB ea, one a copy of the other...or 1 HHD, 1TB, set up in RAID mirror.

[MikeA]

If you are asking if you can have a Raid consisting of ONE drive only, the practical answer is no. You could partition a single drive and synchronize the contents of the two partitions in a raid-like manner, but there is no real benefit. If the drive dies, all is lost.

Raid is a system of managing multiple disks to improve the system reliability (usually) or speed. Minimum drives for the simplest implementation is 2 drives in 'mirror' configuration, and this is called Raid-1. If one drive dies, then the other drive has the identical information, so the system should be able to proceed without interuption.

Raid-0 is where two drives are used to make a single 'striped' volume. When data is read, 2 drives deliver the data, and the speed of delivery approaches twice that of a single drive in ideal circumstances. Downside is that if you lose a single drive, you've lost all of them.

Raid-5 is where 3 or more drives are grouped together in a way that improves read speeds AND allows one of the drives to die without losing data. Each drive in the array contains part of every other drive's data, such that a missing drive can be 'reconstructed' should it die. Write speed for a Raid-5 is relatively slow, because every write creates a parity calculation and a multi-disk write. In any Raid-5 array, the equivalent of one drive's capacity is lost to the parity data, so a 3 drive Raid-5 is pretty much a waste of time. (better to go with a Raid-1 and use the 3rd drive as scratch or hot-spare)

The last Raid array you will hear about is Raid1+0 or sometimes Raid-10. This array has a minimum of 4 drives arranged striped AND mirrored and although half the total drive capacity is lost to the mirror, it's fast for both Read and Write, and can cope with drive failures. (how many depends on which drives die. At least 1, maybe 2 may die without loss of data)

For External drives, make sure the drive interface is up to speed. USB does not cut it except for backup drives, it's too slow. Best alternative would be SATA or removable IDE caddy.

Lastly, on a 2 drive system, think about putting your scratch and page file on the second drive. It can make a big difference in a busy system.

Hope that helps

Mike.

[Johan_Elzenga]

Also remeber that a mirror RAID is not an alternative for a backup. If a file gets corrupted, it's corrupted on both disks of the RAID. If you delete a file by mistake, it's deleted from both disks of the RAID.

[Martin_Doudoroff]

Michael:

As the other responders have outlined, the only way RAID can make your data safer is to protect against drive failure, and only certain RAID configurations will do this. (Other RAID configurations will actually make your data LESS safe.) This stuff is outlined above and massively detailed elsewhere on-line.

The problem is that drive failure is only one way you can lose data. Other ways include equipment loss through theft, fire, or "Act of God", and the most vexing of all, human error.

If you don't have a backup strategy, I recommend you drop whatever you're doing and get one now. There's no "one true way" to handle backup. Unfortunately, you have to think through an approach that will work for you. What basic advice I have:

- The more copies of data you have spread around, the safer you are.

- A lot of us use rotating backups, so that we have two or three versions of any given file, going back in time. You can establish rotating backups through hardware and/or software. With hardware rotation, backup is done to a different hard drive each time, in rotation. (You need two or more hard drives to rotate through.) With software rotating backup, the backup software automatically archives unique versions of your working files to the backup volume. (You need a lot more backup space than working space if you use software rotation, because the backup volume fills up quickly with multiple copies of your working files.)

- It's also safest to have at least one backup off-site, so if your house or office burns down, you've still got most of your stuff, even if it's a couple days out-of-date. This is easy if you work in a separate location from your home because you can keep a backup at the other location. Failing that, use your imagination.

- Automatic is better. If you have to remember to backup, you'll forget, and then something bad will happen. I use software like this, which is highly flexible and provides extensive backup automation: I just set it up and let it go. I only have to check occasionally to make sure I'm not running out of space on my backup drives.

Martin

[ChristianLandry]

I have been using the same setup for the last four years: dual mirror doors G4 with 2 internal drives. I tried the external raid HHD from Lacie (320G,BigDisk) but didn't see significant difference than with the internal second drive. The second drive makes a "real life" difference with Photoshop and allows me to use my ol G4 to this day (though it shows it age ! ). Martin's comment "mirrors" my thoughts but one question remains : I simply use the Backup application that comes in OSX and feel safe with it ( backup to:second drive and .Mac) but i rarely see it recommended in forums, am I missing something