Well the short answer is: don't bother, at least not for external harddisks. I've tried this for quite a while, because at first it all seems to work, with the sbp2 driver and an external harddrive that works via usb as well as firewire. But after a few days, I always started getting errors in syslog and then the device would be offlined and the external harddrive powercycled in order to make it work again. Note: this also happens when using max_speed=0, i.e. making the driver very slow.
For now I've switched to a USB-controller and am using usb-storage, because that, afaict, works more stable.
Today (2007-05-01), we did some preliminary tests with a sil3124 pci sata-card, and that seemed to work with quite good performance (better than the internal SCA-disks anyway). I haven't done any long-term tests of that to see whether it's stable ..
hope this helps someone
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