I took a look through some of my old Apple users guides. I didn't scour them, but here is what I found:
In my copy of "Macintosh II Owner's Guide" copyright 1988 one screenshot shows "Hard Disk" but that seems to be all.
In my "Macintosh System Software User's Guide, Version 6.0" copyright 1988 it mostly has screenshots with "HD20" but has one named "Macintosh" in a screenshot on the cover and another shot with "Craig's Hard Disk".
In my "Macintosh User's Guide" copyright 1992, it very consistently uses "Macintosh HD", though there is one instance of "Hummer" as a hard drive name. This user's guide is designed to be pretty general, and it mentions the Mac Plus, the LC, the IIci and the IIfx by name.
My overall impression is that while hard drives were an optional accessory the naming conventions were all over the map, but once they started shipping systems with built-in hard drives, they came up with Macintosh HD as a consistent name fairly quickly, which seems like a very Jobs thing to do.