I have a 250 G HD connected to my 1.25 Ghz eMac running Tiger 10.4.5 via USB. It has the option to use Firewire as well, but when I try it this way its icon would disappear from my desk top after a day or two or the drive light will flash constantly.

The USB is a solid connection and the external HD icon stays on the desktop with no problem. I understand I can boot from the drive only if connected via Firewire, so that would seem to be the ideal way to go.

Anyone know why this instability only happens with the Firewire connection?

Also, is there anything to prevent having both the Firewire and USB cables connected at the same time? Would this make the connection more stable?

I have 2 external FireWire/USB2 enclosures that have unstable connections via FireWire as well on both my PowerMac G4 as well as my MacBook Pro. I just chocked it up to a crappy chipset. I have 2 FireWire only enclosures (one for a DVD burner, one for a hard drive) and they work perfectly. My experience is if you want to use FireWire, get a FireWire only enclosure with an Oxford chipset if you want to be safe. Not all chipsets are bootable too and manufacturers aren't always forthcoming about whether their enclosures will boot or not.

you may also be able to update the firmware in the case depending on what it is (system profiler should be able to tell you what the chipset is). I've seen this behaviour with some of the older oxford chipsets (pre 3.7). I'm using the 4.0 firmware on all my enclosures now with no problems, but I had this exact same problem on my powerbook with a couple of my firewire cases.

You can probably find the downloads for the latest firmware if you look at http://fwdepot.com .. they are a bit buried but they are there somewhere :)

HTH

P.S. Using both connection types won't help.