So I encrypted my machine (Powerbook Pro circa 2009) yesterday night. Well, let me be more accurate: I encrypted my Volume that has Mountain Lion installed. The hard disk still retains a Snow Leopard Volume. So today I decide to do some update work on the Snow Leopard partition. I'm in Mountain Lion, I go to System Preferences, Startup Disk and select 10.6.8 as my start up disk. I restart the machine, but before it loads up it takes me to a 10.8 screen where I have to select my account and type my password. I do that, but then it loads 10.8 despite the fact that I chose 10.6 in System Preferences previously. Anyway, I go to System Preferences again and I once more select the 10.6.8 as my startup volume. Reboot, everything goes well this time, now I'm in 10.6.8. I do my update work (download email, install MS Office updates). I notice that Dropbox and Google Drive do not work anymore, their folders appears as regular folders even though both applications are running. Google Drive is actually complaining about it. But anyway, I digress. I notice that my encrypted 10.8 volume does not appear in the Desktop. I go to Disk Utilities, it appears as "Incompatible Format" (remember, I'm in 10.6.8 now). Uh-oh, this means I can no longer go to System Preferences and change my boot up volume to 10.8. I have to shut down the machine, restart, and press the option key to select a volume. It does indeed show both volumes: Mountain Lion and Snow Leopard. I select ML, it takes me to the login prompt, I type my password, it accepts it and then disaster, screen goes black, panic messages on white font and the computer restarts, and boots from Snow Leopard. I restart, press option key again, select ML, type my password, and then to my relief the reboot continues smoothly and I'm in Mountain Lion. System Preferences still has 10.6 as the boot up disk, I change it immediately back to ML. If I don't, who knows what will happen next time I reboot my machine. Conclusion: It looks like it's highly risky to have an encrypted volume along with an unencrypted one on the same hard drive/computer. I think that things will be especially messy if one of the volumes is an older OS not supporting FileVault 2. I really thought for a moment there that my temporary switch to the 10.6 meant that I will longer be able to see my 10.8 volume again. I'm now cloning my ML volume with CCC to help me feel more safe.