Is there a difference between raid1 and ordinary hard disk?

You investigated how RAID1 is working already?

well I know its a mirror thing.

what I need to know is, if you take out RAID1 hard drive from a computer, and put it in another computer that is identical but without RAID controller; would it work?

if not, how do you convert it to non-raid drive?

If it’s a software RAID1 and the mainboard is the same it should work (although you get two machines with the same hostkeys), as for booting you need the initrd for this particular chipset. LILO/GRUB needs to be installed on both too.

If it’s a true hardware RAID1, you are most likely out of luck. But it depends on the vendor of the hardware RAID1, what they stored on the disk. Usually they write some extra information to the disk when you add it in their BIOS setup and they can’t work alone.

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what if true_hardware_RAID_controller breaks, and we put in another hardware RAID controller (different manufacturer/model)? would it work with the old drives?

What each controller vendor writes on proprietary information to the disk is not known. In that case you will need to restore all stuff from a backup.

Nevertheless: in certain cases it might work, but I wouldn't rely on it for my system. It’s better to assume that it won’t work. If you have a service contract for a computer system, the service company needs to have some of exactly this type in stock.

well in that case, dd would not be a smart way to backup the RAID1 drive?

A dd of the complete disk would copy only the part of the disk which is made visible to the OS, i.e. w/o the proprietary information. But it’s not guaranteed to work as it depends on the disk layout where partitions start and end. If you have a different disk where you restore the information to, it can fail. More promising would be a dd of the individual partitions which you could save as a file to somewhere and then mount as a loop device. Restoring the complete dump of the partition to a disk might give the same problem like a dd of the complete disk as the disk layout is different.

For a backup I would suggest a real backup application like Bacula.

but what is 'proprietary information'?
all we need to backup is OS with installed software.
Bacula needs installing, and that takes system resources.

The “proprietary information” each hardware RAID card stores on the disk which disk belongs to which array and alike. Imagine having 4 disks for a RAID5 plus a hot spare. How to know which disks belongs to which array? This information won’t be visible by the operating system, as even a RAID5 across 4 disks will show up as one disk with the assembled total size.

The disk image might change during operation. I would use dd only with an unmounted drive or one in read-only mode.

What do you mean by “system resources” - a few megabyte lost on the disk? I would emphasize to use a such a solution, there are even more.

ok, thanks Reuti. the thing I forgot to mention is that server uses Windows. Bacula seems to fully support only 'client daemon' for Windows.

the idea was to shutdown server, take drive out and backup it with dd.
the software we use on it already has backup options. so we don't need Bacula or similar. but it can't backup the OS itself. and they never gave us installation CDs for OS.

Really: forget this idea of a backup. You would always have to copy the complete disk, despite the fact that only half of it may be used. And you need an identical disk to restore it.

Well, this is a Linux forum and so you get suggestions for Linux or BSD like systems at the first place. Nevertheless: you can look into using Retrospect, AFAIK they have a recovery CD in the package.