After I had read a post about someone who managed to boot his Macbook Pro from an external drive using a PPA Int’l eSata ExpressCard, I ordered such a card too. However, it did not work fro me. My external drive would not mount. I never found out why, but after I upgraded to Snow Leopard, I decided to give it a try again, and I found out that my eSata ExpressCard that had been siting in a drawer for ages was now working. I am using the same external drive that I had try about a year ago, and it has now been working perfectly for over a month now. I am even set up my TimeMachine to do backup on that drive, and it is working without a glitch. My MBP is an early 2008 one, just in case it makes a difference.
One thing I have to tell though is that PPA Int’l support is very poor. I got only one reply from them telling me to try the drivers from the JMicron web site, which did not work with OS X 10.5.x. After I have never been able to get anymore support from them. They would not reply to my e-mails.
I haven’t tried though to boot from an external drive using the eSata card.