Trouble is with a mirror as opposed to a backup is that if you get filesystem corruption you risk mirroring that. A mirror at hardware level (Raid 1 for instance) will often use block to block mirroring, regardless of the filesystem. If the whole filesystem screws you could loose both copies but chances are there will be safeguards.

Things like sync should be OK as they mirror at file and folder level. The trouble with automatic syncing is that if you get a corrupt file or folder or a virus, you may end up syncing that as well.

Trouble with having both disks close to each other is that if you have some sort of disaster, heaven forbid, you risk loosing both copies anyway.

Idealy what you would have for any data that you genuinely cannot do without is a mirror of your disk on a hardware raid array. You would then keep a multiple versions of the same files so you can recover easily to a version betweed a day and a week old and then you would have an offsite copy - either on tape or to an offsite agency.

Of course you can make your data absolutely secure and still loose it due to operator error or malice...

As a high availability and distaster planning consultant of sorts (amongst other things!) the first thing I ask clients is how long could they do without their systems, what would it take to re-create the data from scratch and how much are they willing to pay to secure it. They usualy say the need their systems 24/7, all the data back to 1872 and will pay anything for it.

When I tell them how much that will cost the pick themselves up off the floor, realise that they can manage with no system at all for a week and they will only ever need to recover back to last Tueday...

In a nutshell - most things are achievable at the right price.

Cheers

Dave