As an example, consider that the CR-90 Corellian Corvette doesn't come standard with a backup hyperdrive. However, the Cybershop Ship (from Special Modifications) does have a backup hyperdrive.

I don't have special modifications yet, what is the cybershop ship?

If the backup hyperdrive is a replacement/spare tire thing, it can't take up or need hard points. Also, if a ship then lists a backup hyperdrive in the description, would that only mean that one is included in the "trunk" when you buy one?

Are there any suorces that mention the backup hyperdrive other than the Roleplaying games?

The Cybershop Ships is a modified CR-90 with the light turbolasers removed to allow operating rooms, patient care areas, various medical and tech support, and extra berths for all the needed crew added. Along the way it gains back 2 hard points and picks up a backup hyperdrive.