I installed the developer preview of OS X Mavericks, & too many things broke. I can't stay on it & need to roll back to Mountain Lion. Any ways of doing that? Please help!
I originally installed Mountain Lion & Mavericks through the Mac App Store.
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Since the question does not mention backups, I guess there aren't any. What you can try in this case, is to make a time machine backup (e.g. on an external USB Harddrive) of the current Mavericks partition.
Then do a clean install (as suggested by jaume) and then use the migration assistant getting your data back from the Maverick backup into Mountain Lion.
If you backed up your Mac before installing OS X Mavericks please take a look at:
If you want to do a clean install, take a look at:
To reiterate the obvious, you cannot simply roll back to a previous version of the OS. Many parts of the system like iTunes and iPhoto keep backup copies of library files so you can revert back once you handle the OS erase / reinstall.
Other data buckets like iCloud sync and email, calendars, contacts should be exportable or stored elsewhere. You may have to re-implement the settings you choose since they may be changed to a format that doesn't allow a simple copying back of preference files from a newer OS to an older OS.
I had the same issue as you. I was stuck in Mavericks and any time I'd try to boot and select a Mountain Lion installer, it wouldn't work.
This is what I did: