Im trying now for two days and tried everything and I cant boot to Ubuntu on external HD. On OSX Disk Utility I have created new partition on ext. HD and leave it as free space. Then I have downloaded Ubuntu desktop 13.04 and burn it on DVD. I restarted MBP and hold the "c" key to boot from DVD. Everything goes OK. I have installed Ubuntu on this free space partition (I have change it in installation process to ext4 partition type and to / as a mount point, about 250GB partition "/dev/sdb4"). I have also choose Device for boot loader installation" /dev/sdb4, so the same where installation is. I have also created a swap partition (around 10GB). Installation goes fine and everything is OK. Then I have booted to OSX and installed rEFInd which also was successfully. I have rebooted OSX and rEFInd menu appears and also a Linux partition. I have choose it and I get like "No operating system found" and also "There is no bootable device" (something like that). I have tried a lot of things and nothing works.
What Im doing wrong?
My MBP is MacBookPro8,2" and have OSX 10.8.4
Partitions on ext. HD:
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *750.2 GB disk1
1: EFI 209.7 MB disk1s1
2: Apple_HFS External 1TB 497.4 GB disk1s2
3: Linux Swap 10.0 GB disk1s3
4: Microsoft Basic Data 242.4 GB disk1s4
5: 21686148-6449-6E6F-744E-656564454649 134.2 MB disk1s99