@doomgiver, Thanks for your reply.
I have 3 External HDDs connected through 3 different USB ports. Transcend 250 GB, Seagate Goflex 500 GB, both without power-supply and Seagate 1 TB with external power-supply. I pulled out 2 External HDDs and after which I had not experienced the problem so far. I have turned the system on and off about 5 times so far. Keep you updated.
Could this be the reason for rebooting?