If you buy external hard disk and connect both the USB and the firewire (IEEE 1394a) cables to the same external drive at the same time, you could damage the drive. However, it is OK to connect two firewire cables to the same drive at the same time to daisy chain devices.
Interface  Transfer  rate
USB 1.1Â Â Â Â 12 Mbits/sec
USB 2.0Â Â 480 Mbits/sec
firewire (IEEE 1394a) 400 Mbits/sec
if you want to attach your drive after your computer has already booted, you can simply connect the interface cable, turn on power to the drive, and your OS automatically detects and mounts the drive. This is known as hot-plugging.
In a hot-plug situation, it’s a good idea to connect all power and interface cables to your drive, and to
place the drive where you want it to remain, before you turn it on. This helps ensure that you won’t need to move your drive after it’s running.
