Unfortunately, it is not possible to add the 40Gb IDE drive to a PC56-XP that came with a 4Gb Compact Flash (CF) card installed. Due to limitations of the PC56 BIOS, you can use either the CF card or the IDE drive; but not both.
If you originally ordered your PC56-XP with the 4Gb CF card and you want to change to the 40 Gb IDE drive, you can return your PC56 to ProSoft for the changeover. This cannot be done in the field, as it requires specific changes to the BOIS and Operating System which are outside the level of expertise of the typical PC56 user.
If you decide to make the change, be sure to use the Ethernet to back up all of the data that currently exists on the CF card before you send the PC56 to ProSoft. After the changeover, you will not be able to access that data from the CF card on the PC56 any more.