Both versions are disque in French. (presumably disquette for "diskette") Don't blame the French for this.

The fact of the matter is that the spelling "disk" probably entered common use from IBM who invented both the hard and the floppy disk, calling the latter the Type 1 Diskette. Enough people were exposed to the "disk" spelling from IBM usage that it kind of stuck, although in the early 1980s the spelling "floppy disc" was sometimes encountered.