Solution found (partially)!
After weeks of trying numerous configurations, including 3 OS X reinstalls, and 2 recoveries from backup, I have hit upon an acceptable (to me) solution. Mac OS X internal drive has a couple of hidden partitions which require consideration. At the beginning of the drive is the "EFI" or bootloader partition and, I think, at the end is the Recovery Partition, with the OS stuck in between. Point being, be careful when partitioning. I attempted to partition using Boot Camp Assistant so that OS X would take care of these hidden partitions. Unfortunately it didn't work so I ran the LinuxMint install and manually partitioned the internal drive.
I created a small (500mb) Linuxmint /boot partition on the internal disk. The remainder of the install went on the external hdd. Grub bootloader was pointed at /dev/sda and found it's way to the /boot partition. Voila! Using rEFIt the Linuxmint install boots properly every time.
My primary reason for seeking an external hdd install was to save space on my 320gb internal drive. I didn't completely accomplish this but I only used less than 1% of the internal for the boot partition. Unfortunately I messed up the Recovery Partition on OS X, so I created Recovery on a usb flash drive. I can live with that.