I recently replaced the belt on my Twin's disk drive. I then performed a spindle alignment according to this (http://www.famicomdisksystem.com/tutorials/fds-repair-mod/belt-replacement-adjustment/). I have 8 FDS games. Single and dual sided games. The only one out of the 8 that doesn't load is Zelda 2 (error 27). All the rest work fine, including Doki Doki, Zelda, and Bio Miracle. The kicker is that Zelda 2 works on both of my other 2 FDS drives. It's maddening. Can anyone offer advice?
I have a very similar issue - my SMB 2 disk doesn't work in one FDS, but works in my other unit and all other disks work in both. As much as I love it, the FDS really is a piece of crap ;D
Sorry, not much constructive help there, only sympathy. Have you tried tweaking the motor speed slightly? I'd be a little worried doing it in case the other disks stop working, but it might help if you haven't tried it yet.
One thing to test when aligning everything is to make sure that the games save. Try saving for each game and see what happens. You might need to adjust the motor speed, if you haven't already.
ERR27 is CRC error IIRC, so there is possibility, that one of the blocks just corrupted, or... you should align head unit, instead of wasting time on spindle ::)