A little issue with a RAID setup

Question

I have a Western Digital My Book Studio Edition II 2TB external hard disk drive. Internally, it has two 1TB drives configured in a RAID setup. I recently purchased a Drobo and no longer want to use the former for my media storage needs, so I figured I would remove one of the 1TB drives inside it and plug it into my Drobo and use the Western Digital for the purpose of backup.

Removing one of the drives inside (I removed the one labeled ‘B’) was straightforward enough but when I plug it into my Mac, the LED indicator on its face is slowly flashing now (once per second) and it won’t show up in the Finder or in Disk Utility.

Can someone tell me if this is because I disturbed the RAID setup? If so, is there a way to reconfigure it so that it works with just the one 1TB hard drive in it?

I checked the user manual and it says that this sort of flashing of lights happens when there is an over-temperature condition and that I should try to reconnect it after a couple of hours. But since I wasn’t using this hard drive before I did the replacement, I don’t see how it could have heated up.

If anyone could offer some help here, I would really appreciate it.

Thank you.

Answer 1

What is the RAID you used?

RAID1 - Mirroring

If it is a RAID1 (mirroring), you may use the disks separately (emulating the situation when one disk failed). Probably you may not be able to remove disk B, insert disk B in another machine, and use it. On the other hand, keeping disk A on the same machine it was before (thus removing B) and using this disk A will probably work.

In fact, RAID controllers store frequently some data on both disks which they use to know that those disks are part of a RAID and what is saved and how. If you put a RAID disk on another computer, this computer will not be aware of the fact that the disk was used in RAID, so will probably be unable to display the data on it.

On the other hand, if disk A remains, the computer knows it is a part of a RAID. It will think the second disk is broken, and use the first one.

RAID0 - Striping

This is a bad one. Here, some data is saved to disk A, and other data - to disk B. If one disk breaks, you lose all the data from both disks. You may be able to recover some data, but it is not obvious.

In this case, you must not try to remove one of the drives to use on another computer. Instead, connect all the three hard disks to the old machine.

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Removing a drive from a RAID and put it on a separate machine is always a bad idea. Sometimes, you will need to reconfigure the RAID when reconnecting all the drives; sometimes, it will not be possible. Everything depends on the RAID controller you use.

Answer 2

From the user guide, it looks like the RAID controller can only be configured for RAID 0 (striping) or RAID 1 (mirroring); it doesn't have a JBOD (just a bunch of disks) mode. If you left it in the default (striped) config and just removed a drive, it's not automatically going to switch to single-drive mode (which it doesn't have), it's going to think it has a horribly broken stripe set. Essentially, the controller is not (at least as far as I can see) designed to allow what you're trying to do. You might, however, be able to fake it by reinstalling the removed drive, using the WD RAID Manager to reformat it as a mirror (RAID 1), and then re-removing the drive; after that, it should act as a degraded mirror, which is essentially equivalent to a single drive (although you'll get flashing lights indicating the degraded status).