USB Hard disk problem

I am using Windows XP SP2. I have seagate 80 GB laptop hard disk with external casing. When I connetect to usb port, it is detected by OS and shows in the explorer.

But when I transfer some data from internal hard disk to external one, the computer detects off the external hard disk in between the copying. It stops copying and an error message occurs in system tray that the device can’t be detected and data can’t be copied.

I have formatted my internal hard disk and reinstalled the OS with latest updates. Though the problem remains as it is.

I have also checked using different USB ports but it doesn’t make any difference. Another copy option is to use backup software. It works perfectly but it can’t be performed by the OS.