OK OK -- there are previous threads on this, but I will recap.
(From actual experience with clients)
The 6-pin Firewire port (the D shaped one) is prone to loosening, and it is possible to put the plug into the port crooked, or even with a bit of pressure, backwards. (PLEASE don't reply to say "that's impossible 'coz the connector is not symmetrical", it is far too possible to insert it far enough to do damage, with the least amount of wear, or slackening of the sheet metal surround of the port)
When this happens, the 12V power that the Firewire 400 port carries, can arc to the data pins. This fries the Firewire controller in the computer, the peripheral, or both. It doesn't help if one end is 4-pin Firewire (unpowered), if the other end is 6 pin and arcs. USB is not prone to this because it carries only 5V, and the rectangular connector has a large key integral to it which prevents insertion the wrong way, and the contacts are well recessed.
It costs almost $1000 to replace the motherboard in a Panasonic semi-pro camera (ask me how I know...)
So, if you don't want to risk failure, and you have any 6-pin Firewire 400 connections, make it a rule to only ever connect or disconnect with the power off on both the computer and the peripheral.
The 4 port plug doesn't carry power, and I have never heard of a problem with the Firewire 800 plugs. You can argue that the 6 pin port wasn't designed right, and you may be correct, but that doesn't change the fact that there is a risk. It has not been/will not be fixed, because it is part and parcel of that specification.
