When I connect two external hard disks than the hard disk is connected later is not recognized by the computer.. And gives some error that hardware is not recognized... Earlier it used to work perfectly well but now there is problem with it...

Don't really understand,

I connect two external hard disks than the hard disk is connected later

If the machine is running the hard disk is being used

Can you state exactly what process you are going through and when you receive this message and what exactly is the message