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 astray.
One possible hitch right away is that you’ll need to have the Mac App Store application on your drive. This requires that your Mac be running OS X 10.6.6 or later (for more help on this and everything else you need before you can install Lion, read Dan Frakes’ Lion installation guide, particularly Get your Mac ready for Lion).
Assuming you pass this hurdle, the next place where things may go wrong is during the installation process itself. In particular, you may get one of several warning messages.
Install app warnings
This disk cannot be used to start up your computer
Apple offers no explanation as to why the installation may balk here, other than to say it can happen in “some situations.” 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 again means that the Lion installation will not successfully complete. The message can appear even though your startup drive is obviously not acting as a Time Machine backup. 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. This folder, created by and used by Time Machine, is fooling the Install app into thinking your startup drive is actually a Time Machine backup. The solution is to delete the folder.
Some features of Mac OS X Lion are not supported for the disk (volume name)
If this message pops up, you’ll still be able to install Lion. However, as explained by Apple, the installer won’t be able to create a Recovery HD partition. Recovery HD is used for performing “emergency” options, such as reinstalling Mac OS X or running Disk Utility. While you can install and run Lion without this partition, ideally you want to have it. Aside from its troubleshooting value, it’s required if you want to use File Vault 2.
This error can occur if your startup drive is a RAID volume or has a “non-standard” Boot Camp partition. If you have a RAID setup and want to keep it, you have no choice but to forgo Recovery HD on the drive. 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, you 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.
Otherwise, if nothing here has any positive effect (and assuming the drive itself is not physically defective), the solution is to reformat the drive and start over.
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. In this same thread, the company lays the blame on Apple: “Packet forwarding is broken on Lion. Lion either does not redirect the packets (your case) or crashes. This bugs have been reported to Apple long time before the Lion release, but it is still not fixed by Apple.”
If you don’t use Avast, 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.
Redownload Lion?
For installation failures that could be due to a corrupt copy of the Install Mac OS X Lion app, you should redownload a fresh copy of the app and try again. However, if a copy of the Install app remains on your drive (which is likely if you have been unable to complete a successful install), the Mac App Store will claim that Lion is “Installed” and will not permit a redownload. In fact, it is likely to make this claim even if you have never attempted to install Lion after downloading the Install app.
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 Mac OS X Lion app listed with a download option enabled.
Editor’s Note: Due to a production error, we posted a shorter version of this article earlier on Friday. We’ve updated the original article at 2:17 p.m. PT with even more troubleshooting details on Lion installation.