Swapping Hi density drives for low density drives

I thought I had this mastered but apparently I don't, and it's driving me nuts. Background: The PCjr, PC and XT use floppy controllers that support double density data rates. High density drives (1.2MB & 1.44MB) obviously support faster data rates, but also support the slower data rates for compatibility with double density media. A nice upgrade for an old machine is to add a 3.5" floppy drive. Genuine 720K double density drives are pretty hard to find now, but of course 1.44MB drives are all over the place. The 1.44 MB drives have nearly the same pinout as a classic 5.25 double density drive, and they usually have a media sensor switch which forces them to use a slower data rate when using double density media. So I swapped the 5.25 double density drive from a PCjr and replaced it with a 1.44MB drive. The Jr can't tell the difference electrically - it wil just use 40 of the 80 tracks. Everything else will be the same. It ran diagnosics and made all of the right noises, then failed. Hmm. So I took the double density media I was using and cut a copy of DOS 3.3 onto it and tried again. Disk boot failure. Hmm.. bad media. Tried again, different diskette. Hmm.. consistent. To make a long story short: - Original IBM DOS 3.3 diskette: boots and runs fine in any drive. - DOS 3.3 copied onto a diskette by Windows or Linux using a 1.44MB drive and booted in a Jr with a 1.44MB drive: boot failure, but disk is readable. Just parts of track 0 are unreadable. - DOS 3.3 copied onto a diskette by the PCjr using a 1.44MB drive. (Booted first with the genuine DOS 3.3, tried to format /s.) Formatting failed - bad media. (Media is good on other machines.) So I found a genuine 720K drive, put it in the Jr, booted DOS and copied DOS again. Then I put the 1.44MB drive back in and booted from the new diskette. It worked! Basically, it's behaving like the 1.44MB drives in both the bigger PCs and on the Jr can't format the 720K media correctly. And I tried several drives (Sony and Teac) and it failed consistently. A real 720 drive can create a diskette that the other drives can boot from. (The bigger PCs with the 1.44MB drives *dont* have a problem with the copied diskette.) Just whipped out the multimeter. The 1.44MB drive definitely sees double density media - pin 2 is being driven high for double density media where normally it would be low. The controller ignores it either way, has pin 2 was unused back then. Does anybody have experience with doing something like this? I thought that the 1.44MB drives had a media sensor that would slow down the recording rate when using double density media. There should have been no problem coping/formatting double density media. And yet, here I am ... Mike