Retrocomputing is not an easy hobby, especially when it comes to fixing original hardware. Sometimes original spare parts are so hard to find that you need to look for replacements. Recently I was looking for a way to replace the internal floppy drive in my Atari 1040 STFM, and the only reasonable option I found was to use a PC drive. The good news is that PC floppy drives are very cheap and easy to find, and virtually all of them can be used, as long as they fit inside the original Atari case. The bad news is that there are a few quirks you need to handle before you make such drive work. Here are two important pieces of advice to help you deal with it without much stress.
First, floppy drives used in PC compatible computers work as logical unit 1, while Atari drives work as logical unit 0. Some older floppy drive models have jumpers that allow you to set logical unit number, but they are very rare and hard to find. However, on the hardware level, the difference between the two modes is the line number that is used to respond to drive select signal from the floppy controller: those lines are connected to pin 10 and 12 of the floppy drive. So to convert a floppy drive from unit 0 to unit 1 you would need to switch those lines either in the floppy drive cable or in the floppy drive itself - but it's a difficult operation if you don't have proper tools and at least some experience in soldering electronics. Fortunately, Atari internal floppy drive controller (as opposed to PC one) is not designed to work with two drives, and it ignores drive select signal. This means you can safely shortcut pins 10 and 12 in the floppy drive and it will not confuse Atari floppy controller at all.
It is easy to find a floopy drive with clearly visible pin connectors. I used Samsung's model SFD-321B, and soldered a small wire to the pins marked with red dots:
Second, Atari STF/STFM/STE can't handle high density (HD) 1.44 MB floppies. As long as you use single density (SD) 360 KB or double density (DD) 720 KB floppies you are fine, but if you put a HD floppy disk in the PC drive connected to Atari, it will start sending information to the floppy controller which Atari will not be able to understand. Again, there is a simple trick to work around this problem. HD floppies are recognized by the drive through an additional square hole in their casing. Because they can be safely formatted and used as DD disks, you can use a piece of black tape to seal this extra hole so that a photocell in the drive would not activate and a high density disk was treated as low density.
