“.Sony” and the reason why you should never type it on a Macintosh (2002)

“Before it’s opened, you identify a device driver by its driver name; after it’s opened, you identify it by its reference number. A driver name consists of a period (.) followed by any sequence of 1 to 254 printing characters. A RAM driver’s name is the same as its resource name. You can use uppercase and lowercase letters when naming drivers, but the Device Manager ignores case when comparing names (it doesn’t ignore diacritical marks).”

You can see it on bootable HFS CD-ROMs too: they will have a `.AppleCD` at the root.

Reading between the lines (since it isn't justified farther than the above), I think it was just a way to avoid conflicts with file names people would actually use. Maybe also a convention to sort those files "first" when reading a volume, like how people used to append various numbers of space characters to sort their Apple Menu.

[0] https://spinsidemacintosh.neocities.org/im202#im019-001