Booting from a SW RAID

In the previous post I said that you cannot boot from the software RAID array. In fact, this is not true. All the widespread bootloaders, from Windows NT, Linux LILO, and GRUB, would successfully load OS from a RAID 1 (mirror volume).
It should be noted that some further actions must be undertaken for RAID 1 boot volume when you plan to build a RAID.

Since the mirroring technique does not extend to the Master Boot Record (MBR), you must copy the MBR manually between drives in a software mirror. Otherwise, once the primary (boot) drive fails, you will end up with an unbootable system.

Surely, there will be no irreversible data loss - all you need is to just mount the disk into some other machine and read it, but you get no automatic recovery in case the disk fails in operation, requiring a reboot.

Why it is critical to read data off both disks in RAID1?

When using RAID 1 it is needed to access data stored on the second member disk as well. Otherwise it may lead to that a bad sector appears on the second member disk. In this situation the RAID 1 doesn't have redundancy any longer and on top of that you do not know it. Absolutely the same is true for hot spare drives.

Typically one of the causes of this issue is the array being idle. To avoid this situation you can inspect both copies of data while reading. It should be noted that heavily loaded RAIDs do not have such issue.

In Windows, at least up to Windows 2003, RAID1 software implementation usually reads from the first drive, rendering RAID 1 unreliable.