If I clone a HDD to a 2nd, identical HDD using Acronis TIH or similar, can I continue to use the 1st as my bootable drive, disconnect and remove the 2nd, and then reconnect the 2nd in case of failure of the 1st? Will the 2nd boot ok or does it require some kind of MBR, restore etc?
When you Clone a HDD, you make an exact copy (clone) of the full disk, not single partitions. It is usualliy done to make an exact copy of a HDD to a new disk for use then. It doesn't have to be the same size but require the same size or larger disk. You can disconnect it, put it away, and if push comes to shove, you can just connect the new HDD set it up the same as the "cloned" drive (Disk 0 in the BIOS, first in boot order) and it will function just like the older drive. It has the System Files and is the Boot Disk since it is a clone. However if that is a year later, the clone will have exactly what you cloned on it a year ago, not the latest data.
If you Image a Partition, like Acronis True Image you mentioned, or Ghost, you are making an exact image of the original Partition, but it will be compressed to save space (you can choose the compression ratio). Everything will be exactly the same when you restore it but the bits won't necessarily be in the exact same location, when you restore it back to your regular HDD. This is used for Backing Up and Restoring data should things go bad.
Acronis True Image can also Clone a Disk to a new disk for more or less immediate use, like you might do when replacing an older HDD with a spanking new one, but that is not its main purpose in life.
Excellent! Thanks John, exactly what I wanted to know!
I will only be using the clone in case of catostrophic failure of the original HDD and external HD at the same time. I think I'm right to think that this will avoid having to restore system images etc and I can just plug the HDD straight into the same SATA port and start straight away?
I will only clone ever few months (depending on how long it takes) and then store this drive in a secure location. In the meantime I will (as you suggest) make regular backups and images to the external drive which I can use for data recovery or problems with my OS install.
Presumably, so long as I have enough backups to cover the period.....instead of recovering images and then restoring my back ups, I could just put the cloned HDD in and restore the backups to that?