MBP won't boot from FW800, but will boot from USB on same enclosure

I have a Macbook Pro mid-2010 2.66GHz i7.  The boot drive (Toshiba 500GB originally in the machine) is in an external enclosure as I am debugging a problem with the second SSD drive to fail in the last 2 weeks.

The enclosure is a OWC Mercury On-The-Go with SATA inside an FW800 and USB outside.

My problem is that the MBP will not boot from FW 800 on this enclosure, but will boot from USB.   I can take this enclosure to a different MBP and it mounts fine (though I can't try to boot it because the sofware is not compatible with the other MBP).

I have another enclosure with FW 400 and USB, and the MBP will boot from the same disk in this enclosure vie either FW 400 or USB.

Any ideas about what might be wrong, and how to get the MBP to boot from FW800?

[btw, OWC is sending me a replacement enclosure, in case that's the problem, but it seems weird that I could mount the drive on a different machine but not boot from it]
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et01267Asked:
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et01267Author Commented:
I have read that and similar posts on Apple Discussions.  

The problem is that when the external is attached via FW800, I only get the flashing ? Folder after restart, whether I hold down the option key or not.

Also, this machine has no internal drive; the only bootable partitions are on this external, from which I can boot with no problem using USB.

I have already reset my SMC.
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et01267Author Commented:
weird.  Just now I was able to get the machine to recognize the boot partitions on the external drive by holding the option key after power up.  Possibly in my earlier attempts I was not pushing the option key early enough (though I tried several times).

But, without holding the option key, the machine will not find the enclosure.  I suspect that it has to do with bus power not being available at the right time during power up.  I have a power adapter for the external, and when that's plugged in I think it works booting hands-free (though I'll have to test).

I'll split the points up; I may complain back to the OWC guys that their enclosure won't boot "hands free" from FW800 without a power supply, but I'm not sure I have the energy for another battle with an increasingly long list of vendors with broken products.
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et01267Author Commented:
I think the problem lies with either the enclosure or the design of the MBP -- seems like there isn't enough power on the bus at power-up.
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