Fastest configuration for external SSD to drive old iMac?

Hello

I'm buying a fast SSD with firewire 800 enclosure to speed up my 2009 iMac. Said Mac cannot be upgraded to a fusion drive and installing an internal SSD is a pain - so a FW800 it is. May not be as fast as a SATA connection but I think it's the latency on the HD that is the problem...

The next question is what should I put on my SSD. Here are my choices:

- OSX
- General library
- Apps & utilities
- My user library
- Other user libraries

Then I can leave this on the old HD:

- My music and my photos
- Same for other users
- Local data folder, including large mail file
- Local copy of my synched Dropbox

Does this make sense? Do the libraries have to be on the same drive as the OS? Would I get better speed/reliability with a different configuration? Can I daisy chain the OS disk with my photo backup disk (also FW800)?

Finally, do I just drag/copy or start from a clean install of 10.11 on the SSD and use migration assistant?

Thanks!

PS: the original HD is 1TB, the new SSD is 0.5TB. Everything is backup up to a networked time capsule.

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