Here is a great server room security post by Martin McKeay at his Computerworld blog. Martin points out the issues with thieves starting to target servers. But are they stealing the servers for the hardware or the data on the server?
If it is for the hardware, then there are many measures to lock down a server room. Locks, cameras, racks with locks, etc. Martin gives a good rundown of the security measures and points out that taking design of a server room seriously is crucial.
If it is for the data, then the thief has to have considerable intelligence on what servers have the data they want. And that means that security by obscurity is also a valid concept in physical security.
As I have said before, thieves go for the weaknesses. We have to look at security holistically. All areas need to be secured, not just the attack vector de jour.
Vet
