updated 03:45 pm EDT, Thu July 21, 2011
May require shrinking partition or deleting files
Some Mac owners are being prevented from installing OS X Lion, talk on Apple's support forums shows. In select instances users will pick a destination drive and be given an error message, such as "this disk cannot be used to start up your computer," or "this disk is used for Time Machine backups," even though the target was previously used to boot up Snow Leopard. The issue can happen regardless of how much space is on the drive, and whether or not a Mac meets other Lion system requirements.
In the case of the Time Machine error, a potential solution may involve deleting content called Backups.backupsdb, although problems may still arise. In other circumstances, Apple is suggesting running Disk Utility to resize the target partition. "The partition only needs to be approximately 128MB smaller than it was before resizing," the company says, adding that after the Lion install the partition can be returned to normal. Should this not work the extreme solution may be to simply revert to a fresh copy of Snow Leopard before applying Lion.