fstab to keep my dead Main drive inside my '07 iMac from mounting at boot time and dragging the whole system down. I copy-and-pasted this from https://gist.github.com/threebytesfull/968327 because GitHub kept through a 500 error every time I'd try to clone that gist. Props to the original author of this solution.

fstab

# Original gist: https://gist.github.com/threebytesfull/968327
#
# See also: http://apple.stackexchange.com/questions/17264/prevent-bootcamp-partition-from-automatically-mounting
#
# You can find the volume UUID in Disk Utility. Select the volume on the left
# and then File->Get Info. See "Universal Unique Identifier".
#
# Options used below:
#  ro for readonly (can be omitted)
#  noauto to prevent auto-mount
#  noatime to skip writing last access times (performance thing, optional)
#
# Remember to set the filesystem type (hfs, ntfs, etc) too
##########
# Internal iMac HD that's dead
UUID=1B39A092-75B2-357E-97FB-23C082975B80 none hfs ro,noauto