I have a dell laptop and last week it showed blue screen and didn't boot. I had improtant data on in hence I didn't format it and changed to a new hard drive. Now I am trying to connect the old hard drive externally it detectes only D drive & doesn't detect the C drive over that it freezes my laptop. I tried with other laptops & systems too but it's the same. I have important data in both the drives. Please suggest a possible solution to retrieve the data.

Ok, so are you connecting a normal hard drive that's meant to be internal, externally through an adaptor or are you connecting an actual USB hard drive to the computer? It could be that you don't have the pins on the hard drive set up correctly. If it's a normal hard drive, ATA, there should be a schemadic on the top of it for MASTER, SLAVE, and CABLE SELECT. Set the small pin for CABLE SELECT.

The first one is right. It a internal hard drive, externally connecting through an adaptor. If it detects too it doesn't show C drive [when it used be an internal hard drive]content. Only D drive content is visible. After a while computer freezes.