I found out how to fix the crashing modpacks (updates failing). When you start up the launcher, go to the settings icon, then allocate 1-2 GB of ram and check PermGen Size. Restart your launcher and the problem should be fixed! Enjoy!
Edit- I have checked Windows 7 64-bit but I am not getting any launching or updating errors. As of now, this fix is only for Mac OS X. This fix was applied on 10.8.3 with Java 7 Update 17. Thanks!
Leave a comment if it worked for you!