Such a method can work if you are short on hardware. However it should be treated as "dirty hack" and contain some risks and depend on hardware used.
First of all you should know RAID controller can be "soft" or "hard". If your server is built on workstation-like hardware you likely have soft one, also called host-RAID. Check device manager - Intel Matrix or Intel Rapid Storage are host-RAID controllers. This controller does actually nothing, driver and the operating system control all the RAID functionality.
Hardware RAID controller, in short, have non-volatile memory with
data needed to "remember" drive configuration on each start. It also store data on the drive with some sort of parity, used to check what version of data the drive contain and what version of data each block contain.
If you will boot a server without one drive once, the other drive will contain older version stamp. So when you return it back controller will discard the information on that drive and array will need a rebuild. Rebuilding means that data from the drive in "actual" state is transferred to the drive in "non-actual" state. This operation starts automatically or manually depending on what situation controller finds out. Due to its nature and depending on drive size it can take several days to rebuild an array.
First you should prepare:
- Find out what is your RAID type and what utility is used to manage it.
- Ensure your RAID array is consistent. Every controller have an option to check consistency of data. It is a long operation and run in background comparing each data block on both drives.
The outcome of whole operation can be:
- If everything goes well and you have new OS installed you just put backup drive back and RAID controller (soft or hard) will run a rebuild task manually or automatically.
- If you need your backup you should start server without new-OS drive and here you can meet a problem. Some controllers store data version in NVRAM. Therefore it will reject to run on non-actual drive, so you will need to erase controller memory and import configuration from the backup drive. It likely will not happen on host-RAID, but likely will on hardware RAID.
After you used old-OS drive once you can safely add new-OS drive and run a rebuild.
