Originally Posted By: kevs
Tacit, that is a good explanation, did not know any of that.
But, I would be worried that the controller could be screwed up and now I don't have even one good copy! It could screw up everything..

That is a significant problem. If the controller fails, it can be challenging to access your data. Also some controllers are more robust than others when dealing with hardware failures.

I worked with one customer that had a "network storage" unit, that mounted an SMB share on their LAN to use. The drive decided not to mount up on the network anymore. So I pulled out the drive, expecting to just copy off the data, but noooooooo, they had to use some custom in-house file system. Hours of looking later, I found there was actually a company whose only business was recovering data from this specific model of drive, for a high price of course. Further searching found several users had reported that if it finds corruption, it may spend the next several HOURS fixing it before mounting. As in, maybe a day or two. OK, so we let it sit plugged in over the weekend, as it provided absolutely no indication of status or progress while it was in failure mode. It suddenly popped onto the network mid Tuesday. "Copy all your data off that drive, right NOW" We promptly replaced with with a Drobo. Drobo isn't any better for that, but at least if the controller dies you can get a replacement from them as they're still produced.

Then there was someone that brought in a Mac Pro with two internal drives in a software raid that refused to mount. It was a stripe, meaning two drive interleaved together, with no redundancy, so two 1tb drives looked like one single 2tb drive and accessed faster. (read and write) But it had a corrupt directory and disk utility would neither repair nor mount it. NO disk tools I tried could touch it until it was mounted. Can't mount it until it's repaired, can't repair it until it's mounted, Catch-22. Fixing that required my researching Apple's striping methods, and writing a custom program to re-integrate the stripes from both drives onto a single 2tb drive. One drive had IO errors, so I had to plow through those, but it wasn't a big deal. (that was what was preventing Disk Utility from repairing it, the io error was inside a part of the directory) Once on a single drive, (18 hours later) Disk Warrior quickly repaired the problem.

So raids make me a little twitchy. Never consider a raid to be a backup, that's not it's job. Raids have three jobs: (1) protect uptime by allowing you to swap out a failed drive and keep working while it rebuilds instead of being down while restoring from backup, (2) improves read and/or write speed by spreading access over several drives concurrently, or (3) combining several smaller drives to make one larger single logical volume. Nowhere there does it say "backup". Always back up your raids.

Here is an extremely good recount of a large critical raid failure from initial problems all the way through resolution, I encourage anyone interested in raids to watch it:

https://www.youtube.com/watch?v=gSrnXgAmK8k


I work for the Department of Redundancy Department