Some days ago, I updated my Macs to Mountain Lion with the hope of benefitting from its performance improvements. The update process was easy and flawless but as soon as I opened my catalog I started suspecting something was wrong. A quick research confirmed my suspicions: Mountain Lion's update process had completely wiped away the ~/Library/Caches directory which, by default, contains the Adobe Camera RAW cache directory. 8 GB worth of data swept away without even asking: not good and not fair. If Apple wanted to clean its program caches, the installer could have limited to cleaning just those.
Fortunately, I could restore it from my latest backup and I was soon back to work. The bottom line is: if you're upgrading to Mountain Lion, you'd better backup your Adobe Camera RAW cache directory, unless you don't mind Lightroom recreating it from scratch.
If you still don't know your cache size (chances are it's still the default 1GB) or your cache location, you can check it in the Ligthroom preferences (File Handling pane):
|Lightroom Preferences - File Handling|
You should increase the cache size to increase the number of image data that can be stored: that depends on the number of RAW images in your catalog. In my system, I'm observing an average 1 MB worth of data per image. You can also change the cache location in case you prefer storing it closer to your catalogs for backups' and "visibility's" sake (by default the ~/Library folder is hidden in Finder).