RAID, turning off one drive

I can't answer your question directly, but I'd not recommend what you're considering.

There are different types of RAID setups. The one I think you're talking about is called a +*Mirrored RAID set+* or +*RAID 1.+* The same data is written to two (or more) disks, so if one fails, the other has all your data. I've never used this on a Mac, but yes, the whole point is if one fails, you'd no doubt see an error message, but your system keeps right on working.

This is normally used for a primary drive, not a backup drive. If you want dual backups, it's much better to use two independent drives, with two independent backups. One excellent strategy is to use one drive with TM for a week or so, then take it to your safe deposit box, relative's house, workplace, or other secure off-site location and swap it for the other one. About all you have to do is use the +*Change Disk+* button in TM > Preferences, and try not to go more than about 10 days between swaps.

Another strategy is to use TM with one drive, and a "bootable clone" to another (or a separate partition on the same drive). That way, you get the advantages of both types of backups, including the ability to boot and run from the "clone" when your internal HD dies, rather than wait for it to be replaced and restore your TM backups to it.

It is possible to move the backups from the existing TM drive to the new one, so TM can just keep doing backups (without starting over). See item #18 in the Frequently Asked Questions *User Tip* at the top of this forum.

The one I think you're talking about is called a Mirrored RAID set or RAID 1.

Not at all. You cannot use mirroring with different size drives (think about it - you can't mirror 1TB of data on the large drive onto the 300GB of space on the small drive).

What the OP wants is RAID 0 concatenation which can work with different size drives (although it's not recommended).

In any case, I would NOT use RAID 0 for Time Machine.

With RAID 0 there is NO protection against disk failure. If either disk fails then you lose all the data on both drives. This is not recoverable (at least without some expensive professional disk recovery assistance).

This is doubly bad when using external drives, where a loose or pulled cable may disconnect a drive.

It's triply bad when used for your backups since any disk (or cable) failure means you've lost all your backups.

You should bear in mind that ALL disks will fail at some point. If you're relying on having two disks work for your backups then you have doubled your chances of losing your backup data through disk failure.

So my advice is just keep the 1TB for Time Machine. Repurpose the 300GB drive if you like (media storage? bootable copy of your OS? whatever), but don't try to combine your two external drives into one RAID set for backups.

Camelot wrote:

The one I think you're talking about is called a Mirrored RAID set or RAID 1.

Not at all. You cannot use mirroring with different size drives (think about it - you can't mirror 1TB of data on the large drive onto the 300GB of space on the small drive).

Yes, I misread the intent. I was distracted by the question about what happens when one is powered-off, and was recalling many years ago when a RAID drive failed on one of our IBM midrange boxes, the replacement was on long backorder, and we had to use a larger one temporarily (you can do it, but the extra space is wasted).