We upgraded a Mac from Lion to Mountain Lion. After launching the Mail program, it says "Migrating" but it takes quite a while until the progress bar goes to 100% then it hangs.
Workaround (may not be a fix):
I do not have sufficient time to diagnose this completely but the reason for the migration is that it needs to copy all of the files in the Mail storage from one location to another. This is likely for a better security.
- First, be aware that a lot of people do not manage their email. They simply have thousands and even millions of email messages, attachments and even undumped email trash. So this will take quite a while. I would do this over night to see if it was just taking time. Be sure that you change the computer sleep time to "never."
- One possibility is that Microsoft Entrouge may be affecting it. I know a good chunk of people do not use Entrouge so in our case, I just dumped the entire Entrouge in the trash.
- Next, restart your Mac in the Safe Mode. Note that this can take about 10 - 20 minutes of its own time before it starts. This is because the Mac needs to do a process of file system check before allowing you to use it in the Safe Mode. Do not panic. Just let it do its thing.
- Be sure to set the computer sleep time to Never. It will still be several hours of processing!
- Launch the mail, and see if it starts, head for your favorite club, dance all night, come home sleep whatever left of the morning. When you wake up, do not go to the computer, make coffee or tea, take shower, dress and go back to your computer... and voila! I think your computer is showing the usual email list.
- Note: Please do not post a comment if it does not work. Instead call Apple for support. I do not work for Apple and I myself do not use Mac Mail instead just check my email on the web or I use Nexus 7 Android tablet and iPhone. Perhaps you would want to start using web email just in case this sort of thing happens in the future. If you want to manually clean the email data, they are stored under ~/Library but you would want to search such topic using your favorite search engine provider.