How do I avoid the “S to Skip” message on boot?

Answer:

You should add the option nobootwait to your /etc/fstab. So that it looks like:

UUID=1234-5678 /osshare vfat utf8,auto,rw,user,nobootwait 0 0

From fstab(5):

The mountall(8) program that mounts filesystem during boot also recognises additional options that the ordinary mount(8) tool does not. These are: bootwait which can be applied to remote filesystems mounted outside of /usr or /var, without which mountall(8) would not hold up the boot for these; nobootwait which can be applied to non-remote filesystems to explicitly instruct mountall(8) not to hold up the boot for them;