This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
I did some magnetization tests on Double Density (800kB) and High Density (1.4MB) diskette surfaces. The tested DD surface produced more than twice the deflection of the tested HD surface. Clearly, there is a big difference in signal levels required to reliably store data on HD vs. DD. In fact, 5.25" HD (1.2MB) diskettes will not work at all on Apple Disk ] [ drives. The 3.5" HD's may work fine on your 800k drives; or, they may just seem to work fine. Either way, there's no question: a drive optimized for DD will not be optimized for HD. If you'd rather not 'roll the dice' on your software collection, stick with Double Density diskettes. By: Rubywand, George Rentovich, Mad ATARI user alternate, Joel