MacOS – How to reinstall Mac OS X (Lion) without erasing data

You can re-download Lion even though it has already been installed by holding down the option key before clicking Purchased.

Do not release the key until after the option to Install appears alongside OS X Lion.

The workaround may be more reliable if something other than Purchased is in foreground before alt/option-clicking Purchased.

Burn the installer to disc, then delete it when you're done with it.

This is a broad question in which you need help defining a strategy.

If you want a truly clean start, yes, I would consider these steps.

• Back up your entire system to a disk image file on an external drive using Carbon Copy Cloner or SuperDuper!

• Check all your crucial system settings on the startup disk, such as Internet connection settings and network volume connections. Un-register iTunes, the App Store, and any other applications with copy-protection that need to be un-registered.

• Erase your system hard drive, make a clean install of Snow Leopard, download all Apple updates and security patches.

• Then re-connect the external hard drive, mount the disk image of the backup, and manually drag over only your documents, not any system settings.

• You will then need to laboriously download and re-install all your third-party applications and App Store purchased applications, and re-register them.

• You must then decide what configurations and files from your /Library/ and ~/Library/ need to be migrated from your old disk image to your new system. Making these decisions one-at-a-time rather than using Migration Assistant will ensure that you avoid "cruft" or "useless clutter".

• Make sure that you retain your backup disk image for quite a while so that you can go back and get more data from it if you discover that you have overlooked something that you need, like fonts or folders in ~/Library/Application Support/.

I underwent this process myself when I upgraded from Snow Leopard to Lion. I made this decision because my system had been migrated from Tiger through Leopard to Snow Leopard and had many years of "cruft" that I wanted to be free from, particularly some third-party KEXT files where I was not certain what was working correctly and what was not. But going this route takes a tremendous amount of time and attention to detail. You could spend more than one full eight-hour day working with this if your system is as old and complex as mine is.

So weigh your options and decide what you want to achieve, and how much time you are willing to devote to it. But make backups of everything first, and retain those backups!