Default Mount /data read only

I am doing forensic acquisition of / in my Android HTC Evo Design 4G device. I have rooted it using k1mu saferoot. Saferoot has related log file resides in /data/data...

My purpose is forensic so I thought to mount /data read only. When I issue mount command for /data in adb shell it says device or resource busy.

I am able to mount /system read only.

What could be the issue with /data? How can I do this?