Kubla - You asked "i was thinking if i used a hard drive with only firewire then i'd need a dac with firewire. Did i miss a step? Does the dac have to go back into the computer processor first?"
The hard drive and the DAC have nothing to do with each other. They do not communicate at all in any way.
In over-simplified terms, the computer "calls" the hard drive to read some data from the drive - in this case a song. The data comes to the processor (CPU). In this case the CPU sends these 0's and 1's (which are audio data packets) out to the DAC.
You could have a USB or SATA or Firewire or Ethernet or Fiber or SCSI external hard drive. You can have a DAC in any of these flavors. It's just data zipping around.
The DAC does not depend on the CPU to do anything but send it the data that the client wants to hear. In all of the above mentioned formats there is some checking between the CPU and the source or destination to ensure that all the packets have been sent/received - if any packets were dropped, the computer will send them again.
Specific to the Glyphs - the exact same hard drive can be used with either USB or FW. It is a question of what controller is used. The controller is a chip that manages the dialogue between the drive and the CPU. Or the DAC and the CPU. As Drubin points out, in many cases an external hard drive is equipped with both kinds of controllers and plug sets (the cables are different.)
USB comes in two flavors - 1 and 2. 2 is much, much faster then 1. It's very rare to find USB1 on a hard drive, though it is still used for keyboards and the like.
FW comes in two flavors as well - 400 and 800. As you might guess, 800 is twice as fast. Both of these are fast enough for audio.
USB2 is faster (transfers more data in a given amount of time) then either FW format. FW is somewhat more efficient at transferring very large files such as those associated with graphics, digital photography and video. By comparison, audio files are very small.
The Glyph price difference is most likely associated with capacity or some other difference in capability.
The choice of USB or FW does not impact the quality of the DAC. You will find that the higher end audiophile oriented DACs tend to be USB, while the professional audio/studio DACs tend to be FW.
Hope this is helpful - keep us posted
