I tried using 1.44 MB disks on a 720 drive. No luck. Not sure if it looked like it worked and then didn't or if it was a no-go from the get go, but it just didn't work.
According to an article I found 1.44MB media needs to be demagnetized by a bulk eraser before they can be formatted at 720KB.
Depending on the age of the mechanism density can also be set either 720KB or 1.44MB by switch/jumper, or automatically by the HD sensor.
Later 3.5" mechanisms may also have dropped support for 720KB media as a cost cutting measure, eliminating the sensor for HD media recognition and configuring the mechanism to operate only in HD mode. I did say my 1.44MB Sony mechanism is from the early 90's.
In IBM-compatible PCs, the three densities of 3 1⁄2-inch floppy disks are backwards-compatible: higher density drives can read, write and format lower density media. It is also possible to format a disk at a lower density than it was intended for, but only if the disk is first thoroughly demagnetized with a bulk eraser, as the high density format is magnetically stronger and will prevent the disk from working in lower density modes.
Edited by BillC, Sun Mar 17, 2019 8:19 PM.