Found this interesting video in Youtube. It talks about the Google efforts on enforcing security across its data centres. The key security measures’ they are following include:

• Strong physical security controls such as identification cards, manned reception, visitor agreements,CCTV surveillance, Biometric access etc..
• Utility controls such as Power Generators, Fire detection systems and automated data movement based on fire alarm
• Backup seems to be on tapes, which is bit odd from a highly advanced technology company.
• Watching the video, you can see how Google is maintaining the Hard Disk inventory and tracking. I presume it is the use of barcodes and technology which helps them do it.
• Hard disks are managed within the data centre. Hard disks which are no more useful are crushed and then shredded into pieces which gives pretty good confidentiality in terms data protection.

These Google data centres support the Google services such as Google Apps, Gmail etc..