I know Mountain Lion has some cool new features that Lion did not have, but will it overwrite any of Lion's functionality?
MacOS – What features from Lion will be lost when upgrading to Mountain Lion
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Related Solutions
Here are the things you'll want in case things don't go well with the install over your internal drive with your apps, settings and files:
- An external USB drive with a Recovery HD (4 GB is plenty for this)
- Time (Downloads could be slow and error prone today)
Dive in is my advice given your recent hardware, speed of the machine and storage and the framing of your question (you've asked the right details to be aware of the potential downsides).
Do try booting to the recovery USB drive once and see that it shows your Time Machine drive as a valid place to restore Lion. I have had people find one USB port was dead when they tried to use both - I know this is paranoid, but update time is when most people find out about issues (software corruption and hardware problems) since it changes things at a very low level. Machines can be infirm enough to keep running and fail when faced with a re-install or migration of data that a major OS upgrade entails.
If you were paranoid, you could also add another external drive with 20GB free to your list of things needed. Just installing a clean version of Mountain Lion is always safer than installing onto a system with data and settings. By migrating the contents over to be sure all your third party apps are ready for the new OS. This is a nice alternative to making a bootable backup.
Re-doing the installer a few days later on your internal drive is easy work (or using Disk Utility to erase the internal drive and "restore" the external image to the internal drive doesn't take that long for Air sized images.
The Time Machine backup should do a pretty good job keeping all of your documents and most of your settings and should be backwards-compatible*, but I recommend making a complete, bootable backup of your previous system so that you're guaranteed to be able to restore your machine to exactly as it was before updating.
I'm a fan of Carbon Copy Cloner (free trial) for making the backup. Back up to a clean disk or partition using CCC and tell it to copy everything. It will also help you clone your Lion Recovery HD onto the backup disk.
Once the backup is done be sure to try booting from it to make sure everything worked.
*Should be, but I haven't personally tested.
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Best Answer
Of course. In adding features, the behavior of previous features changes, not always for the better.
If you find features that were present in Lion but missing in Mountain Lion, please edit this answer to add your contributions to this list.
Mountain Lion drops support for some models of Macs supported by Lion.
Clicking the top right corner no longer triggers Spotlight (Spotlight's menubar icon is still present, adjacent to the new Notification Center icon).
The battery indicator in the menubar will no longer offer to show the remaining time.
X11.app is included, but that app alone no longer provides X11 functionality. When the user first runs X11.app – or an app that requires X11 – a dialogue guides the user to Apple's article HT5293 about XQuartz, which provides the functionality.
Mail.app and Safari.app drop support for RSS Feeds.