Beginning in 1995 with the PowerBook 190, Apple laptops shipped with an expansion bay for both Apple and third-party drives. The drives were “hot-swappable,” meaning that the user could pull out one drive and replace it with another without restarting the laptop.
This PowerBook Floppy Drive Expansion Bay Module from 1995 is an early example. This module works with the PowerBook 190 and 5300-series PowerBooks. This example is from one of the PowerBook 5300-series laptops in my collection.