Server 'Dies' with USB or Firewire drives

Okay this has me stumped.

We have a 2TB Audio server which has several folders on it. These folders are mapped to drive letters on around 15 computers with each computer having the same drive mappings for the same folders.

All of these computers are networked together via the same switch i.e. all computers that need access to the server are connected to the same switch that the server is connected to.

All day long these computers are constantly writing and reading files to and from the server without any problem.

In other words everything works fine, no problems.

We then have one PC that again is connected to the same switch as all the other PC's and has the same mappings to the relevant folders as all the other PC's which we use to make backups of certain files and folders to external Firewire and USB drives. We can copy data OFF the server onto these external drives via this PC without any problem.

However if we have audio on a removable drive (Firewire or USB) that we want to copy back onto the server (these will be new files i.e. we aren't overwritting existing files of the same name) then when we do this from this computer it slows the entire server down to a crawl. By slowing down the server I mean that if you try and use the Server Windows runs very, very slowly i.e. you try to open up Adminstrative Tools and it takes 5 minutes just to load the first screen. Also all the other PC's that are trying to write to the server or read from it keep getting errors when this happens.

Bizzarely the same thing happens when we connect one of these external drives directly to the back of the server so we know it's not a fault on this other PC.

Server is a Dell PowerEdge 1600P with 12GB of RAM and the drives are defragged every night and our network is a Gigabit one.

Any ideas why this might be happening