Doesn't the RPM need adjustment when using a HD drive as DD? Please excuse me if this was already posted
That's not a simple question, believe it or not.
To understand the issue, a useful bit of information is that 1.2M 5¼" drives have their origin in Japanese PC98 equipment, where they're pretty much just a reduction of the PC98 8" drive (there is also a PC98-compatible 3½" drive that spins at 360RPM, just like the 5¼" and 8" drives). The idea on the PC98 machines is that the recording format is unchanged, regardless of the medium size.
The western 5¼" tradition is a bit different, starting with the Shugart SA-400, with 35 single-sided tracks recorded at 48 tpi on a disk spinning at 300 RPM. The Micropolis-origin 100 TPI drives were intended to come closer to 8" with 77 tracks, but they're mostly a historical curiosity. At some point, as things got better, a "quad density" 96 TPI format was introduced.
Early on, a number of makers decided that a single drive model could satisfy the needs of the 96 tpi world. So, instead of selling, say, Teac FD55F (720K) and Teac FD55G (1.2M) drives, a Teac FD55GF drive could satisfy both markets (I do have some 1.2M only drives on the shelf). There were two ways to accommodate this.
One way was to have a "dual speed" drive, where the drive spindle speed changes according to the selected density. I still have a couple of PC floppy controllers with jumpers to accommodate these drives.
The other way was to keep the spindle speed at 360RPM and add a 300KHz bit clock to the controller, rather than the normal 250KHz one used for 360K drives.
Most Teac 5¼" high-density drives have jumpers that allow the drive to operate in either mode.
So there's your answer--you can change the spindle speed (which almost nobody today does, thanks to the original PC AT, or you can change the bit clock, which everyone does.
Sort of a "Don't raise the bridge, lower the river approach"...