Disk Not Showing Up in Startup Disk

10-11  Source: Network gathering  Views:22 

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I have a mid-2011 MacBook Pro with two partitions on the internal hard drive. One has 10.8.5. One has 10.6.8.
When I boot from the 10.6.8 Snow Leopard disk the partition with 10.8.5 no longer shows up as a bootable volume in System Preferences>Startup Disk. It just disappeared. I can reboot and hold down the option key and then I can choose either partition and boot into it.
But does anyone know how I can get Startup Disk to recognize the 10.8.5 partition again? Any suggestions?
Any advice would be much appreciated. Thanks!
Are you using FileVault on your 10.8.5 partition?
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