Mac OS X Lion is easier to install than any previous version of Mac OS X.
You don’t have to boot from a DVD or flash drive. Just download the Install Mac OS X Lion app from the Mac App Store and let it walk you through the remaining steps.
Still, things can—and occasionally do—go awry.
Install app warnings
While attempting to install Lion, you may get a warning message:
This disk cannot be used to start up your computer
Apple offers no explanation as to why the installation may balk here. However, Apple does provide a solution: Restart your Mac, launch Disk Utility and, from the Partition tab, slightly reduce the size of your startup partition (by as little as 128MB). You should now be able to install Lion. When done, you can restore the partition to its previous size. If this resizing trick fails, you will likely need to reformat the drive.
This disk is used for Time Machine backups
This message can appear even though your startup disk is obviously not a Time Machine backup drive. As covered in an Apple Support Communities thread, the likely cause is an errant Time Machine-related Backups.backupdb folder at the root level of your drive. The solution is to delete the folder.
Some features of Mac OS X Lion are not supported for the disk (volume name)
This can happen if your startup drive is a RAID volume or has a “non-standard” Boot Camp partition. If Boot Camp is the cause, the solution is to reformat your drive as a single partition. Next, reinstall Snow Leopard followed by an update to Lion. Now run Boot Camp Assistant to create a Boot Camp partition. Lastly, restore your remaining data from backups.
If you can’t or don’t want to reformat, an alternative is to install Lion on an external drive. If you ever need to boot from Recovery HD, do so from the partition on the external drive.
Install failed. Mac OS X Lion couldn’t be installed, because the disk (volume name) is damaged and can’t be repaired. Click restart to restart your computer and try installing again.
If you see this unwelcome message, chances are slim that trying again will lead to success. The same is true if the Install application simply stalls before completing. Some users report that “zapping the PRAM” is sufficient to get past this roadblock. Otherwise, according to an Apple Support Communities thread, the fix is to boot from a Snow Leopard Install disc/drive and select to Repair Disk from Disk Utility. Assuming repairs are successful, try the Lion installer again.
Internet connection lost
Although not likely, you may lose your Internet connection after an otherwise successful install of Lion. If you use avast!’s Web Shield, the solution is to uninstall avast!. There is a known conflict between avast! and Lion. As posted in an avast! Forum thread, a supposed bug-fix update, still in beta, is available.
A more general potential fix is to turn off the Mac OS X Lion Firewall (in Security & Privacy System Preferences), and delete the firewall-related com.apple.alf.plist file in /Library/Preferences (via Terminal if needed). After a restart, you can re-enable the Firewall.
For installation failures that could be due to a corrupt copy of the Install Mac OS X Lion app, redownload a fresh copy of the app and try again. However, if a copy of the Install app remains on your drive, the Mac App Store will claim that Lion is “Installed” and will not permit a redownload. To work-around this, delete the the Install app. Next, from the Mac App Store app, hold down the Option key and select the Purchases tab. You should now see the Install app listed with the download option enabled.