So the Famicom Disk System is really hard to get up and running again for a number of reasons. The most common reason is that the rubber belt needs to be replaced. This is a relatively easy and well documented fix. But after you get the belt on, the system still usually doesn't work. You will often get Errors 21, 22, or 27 when trying to get it to read discs. It can take hours of tinkering to get past these errors, and even at that point, some discs will load while others will not. I thought I'd make a post about what I have done with much success. I've gotten about 5 systems up and running using the process below, and it gets easier every time.
The first thing you want to do is adjust the motor speed. Use a thin, flat-head screw driver and insert it into motor as depicted above. If 10 is all the way counter-clockwise, and 0 is all the way clockwise, set it to around 7. It seems to be able to read most discs at that speed.
Note the little indention on the spindle. This is a cause of much consternation, as you must align the tip of the little black grub screw perfectly into this little hole. It can be tricky because it can't be too tight, too high, or too low. It has to be perfect. In my experience, if it's too tight, you will get a lot of Error 27s. I'm not sure if this being misaligned is THE cause for error 27s, but that's what I have observed. But I've also seen error 27s with bad discs, so who knows... You may have to realign it a few times. It's my personal belief that this is the source of the problem most of the time. If you try many times and it still doesn't work, you can try adjusting the drive head as described below.
The next thing you can try doing is adjusting the drive head. To do that, you need to turn the screw above 1/8th turns at a time to see if it makes a difference. Be careful not to strip this screw! If you strip it, game over.
The ideal position I have found is shown in the picture above. You want to have the top of the gold screw positioned slightly over the silver circle below it. While you're at it, swab the little silver thing with rubbing alcohol and let it air dry.
Lastly, there's one more grub screw which you can try adjusting. I don't know what it's supposed to do, but I always loosen it and re-tighten it just for kicks.
By the way, it is possible to use a drive from the standard red FDS system and transplant it into a Sharp Twin Famicom. On the Twin Famicom system I had, the drive head screw was stripped and therefore the drive was unfixable. So I put a regular FDS drive in there and it works great.