Please explain the connection in more DETAIL.
What "case" are you referring to?
Why are you booting from an external drive?
If you can boot with the hard drive connected then both the drive and laptop are fine. Your problem appears related to this "case" you refer to.
I refer to the internal hard disk, as you know there is a metal case on it. Once I got the error that reseat the internal hard disk, we removed the inside hard drive and checked, latter removed the outer case and then connected the drive, it booted fine. but as we fitted back the case, same error started to appear. So why this issue.