The Replay does not require a Physical Harddisk as such to primarily boot up not a floppy disk. it uses whats known as an SDcard which can contain both in Amigas case for example, a Harddisk file or .HDF or alternatively a Floppydisk file or .ADF. The system or replay gets told to use one or both of these to boot up on in software instead. See this link for further information. Floppydisks also in Amigas case, can be loaded into upto 4 drives like an original Amiga did ie: df0,df1,df2,df3. These can then be used to either boot on just like per usual booting up a game etc or demo, or loading a Workbench. Its alternatively able to be setup so as it can boot on the .HDF Harddisk file to automatically load to the workbench. See this link for further information.
The Replay1 68060 Daughterboard also had a real floppydisk connector and as such, it can be switched so as to be made to boot up on this instead via the OSD. This also has the possibility to use a 3.5inch SDcard Floppy device as well or (SD HxC Floppy Emulator) or (Gotek USB Floppy drive) See this link for more information.