My best guess is that, when booting any of the Workbench Floppys (and many games apparently fall into this category), they will always Mount any drives they find, including the hard drive.
Harddrives are mounted by the Kickstart before it even checks for floppy disks. You cannot avoid it unless the harddisk controller has an off-switch.
However, my hard drive had the International FS which 1.3 didn't have a driver for, thus generating the "insert volume LIBS" prompt.
This is not the reason, at least not completely. FFS Intl. does work on Kick 1.3 if it is installed correctly on the harddrive.
To be sure, I partitioned the new hard drive as FFS, and "unchecked" FFS when I Quick-Formatted it.
This is not a good idea. By unchecking FFS you made it OFS. OFS has a very small partition size limit (8 MB or so). If your partition is bigger it will corrupt sooner or later. And it is sloooow. That's why FFS is called
Fast File System. You should keep FFS, only uncheck Intl and DirCache.