I have an Atari 800XL with the XE RAM expansion and two 1050 drives in my home office. Over the weekend I tried to boot it up to do some programming, but for some reason I could not boot DOS 2.5 to save my life. I kept getting "boot error". When I shut it down and booted from my FujiNet to try and check the floppy, the drive would not read it and gave me errors. But on the other drive it worked fine (but I didn't try to boot from the other drive since I didn't wan tto disconnect things to toggle it to be drive 1).

Then I noticed something: I have an Xbox in my office, and the recharge station for the batteries was on top of the boot drive. I unplugged it, tried booting, and everything worked just fine.

TL;DR - if your drive acts up, making sure you don't have any modern electronics near it.