I am running dual boot on my iMac. I upgraded my OS to OS X Yosemite yesterday,and now I can't boot into Ubuntu. How can I fix my issue?
If your Ubuntu partition is still there, you probably need to install a rEFIt (or an equivalent) so you can choose your Ubuntu partition.
This gives the simplest solution for me.
and you will see something like:
$diskutil list /dev/disk0 #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *251.0 GB disk0 1: EFI 209.7 MB disk0s1 2: Apple_CoreStorage 175.6 GB disk0s2 3: Apple_Boot Recovery HD 650.0 MB disk0s3 4: Microsoft Basic Data 70.3 GB disk0s4 5: Linux Swap 4.2 GB disk0s5 /dev/disk1 #: TYPE NAME SIZE IDENTIFIER 0: Apple_HFS Macintosh HD *175.3 GB disk1 Logical Volume on disk0s2 41AF7850-5594-4959-A135-582BF79FDE46 Unencrypted
diskutil corestorage revert /dev/disk1
And reboot, refind now works again.
I got the solution from the related question here: https://unix.stackexchange.com/questions/150670/install-refind-on-os-x-10-10-yosemite/164614, you can try other solutions there if this one fails.