I'm having the exact same thing happen in my system, but with one wrinkle - no Time Machine involved, just an additional 2TB HDD that I use for document/media storage. I use it with both my ML and Win8 installs (on separate SSDs). Everything was working great with booting to both and accessing the drive from both. Then yesterday I get the hung text-spinner (mine seems to like '-') and can't boot from either Chimera HDD or Unibeast USB. If I unplug the 2TB storage drive, boots just fine.
I also found that if I unplug the drive, boot to Chimera, then plug it back in, everything works fine. Not the most convenient approach, but it gets me to a point where I can work on the problem at least.
Other findings - did a disc scan/repair in both Win8 and ML, nothing found, no changes. If I go to MB boot selection, I can boot to Win8 just fine. I was using about 200GB of the 2TB drive when things went south.
My next steps - Backing up my storage drive to external, trying a repartition, then moving things back slowly to see if there is a trigger point at which this seems to happen.
Maybe this will eventually reach critical mass and trigger a bug fix...