Exactly what it says on the tin. I'm getting Error 27 on my games, and although I managed to coax Castevania into working by retrying again and again, it only worked once for my Zelda disk, so now I'm thinking about cleaning the disks.
Probably not the best idea to do that because the disks are some of the most fragile things I've ever seen. Error 27 is a calibration issue so that's what needs to be done. When I replaced the belt in my original disk system I followed this video: https://www.youtube.com/watch?v=fOY1G3Sg11Q At around the 23 min mark he starts showing how to calibrate it. I don't know if this is technically the best/right way to do it, but it worked for me.
There's also this guide as well: http://www.famicomdisksystem.com/tutorials/fds-repair-mod/belt-replacement-adjustment/ but it didn't work for me so I don't know if you'd have success with it. Really it all comes down to if you're down for taking the thing apart.
I'll leave this here too just so you have a list of the errors: https://www.famicomworld.com/forum/index.php?topic=7142.0