Install and run Linux on partition of external HD? Is it possible to install and run Linux on a partition an external HD in OS X?

Almost all the instructions I've found are about installing to the entire external HD, not to a partition.

I've install rEFInd on a Core2 Duo late 2007 iMac and dd'd a Linux .iso to a FAT-formatted partition on an external FW 800-connected drive, but the drive disappears from Finder and does not appear as an option in rEFInd when the iMac is rebooted.

I have to change the partition format back to OSX Extended (journaled) via selecting Disk -> Partition -> click on the particular partition in OS X 10.11.6 Disk Utility, and rename it to see it again, which of course wipes out the dd'd .iso. Duh!

Any ideas or is running Linux from an external HD partition impossible?
I think it is possible but you must partition/erase that partition as bootable.