Hi
If your laptop is modern AND has a DVD drive (which has a SATA interface)
then you can get a tray which can hold a 2.5" laptop hard drive and fits in the space where the DVD drive is currently located . There are different versions depending on thickness of DVD drive tray and hard drive
example
http://www.ebay.co.uk/bhp/optical-bay-hard-drive-caddy
You can use all data files on the old hard drive BUT not the program files.
or as COL Geek says you can get usb to sata Trays for external drives.
regards
Mike Barnes