leyima

i wanted to swap HDDs on two laptops. one a compaq with HDD 12 GB and a Dell latitude CPX HDD 20G. i was running out of disk space on the compaq so i decided to swap hard disks. i fixed the dell HDD on the compaq and reinstalled win xp. on putting the HDD of the compaq on the Dell it will just not load. i fixed it back on the compaq and it worked. so i decided to fix back the HDD of the Dell. on doig this and restarting, i get this message 'a read disk error has occoured. press ctrl+alt+del to restart. this message still comes up even after restarting. but when i fix the HDD back on the compaq it comes up. what do i do?