Spent quite a bit of time with Apple Enterprise today and learnt quite a bit regarding this issue.
First of all, I run a Mac Mini Server, which complicates the issue quite a bit. For those of you running server app, Mac OSX Servers are not truly designed to sleep (server processes actually block sleep by default) and Apple recommends you do NOT sleep your server computers. This by itself may be related to why external disks are not sleeping.
We tested both G-Technology drives on my system (Firewire and Thunderbolt) and interestingly after 30 seconds the Thunderbolt drive DID spin down, while the Firewire drive continued to spin. After extensive investigation, we found that the issue was actually related to SMB2, which Mavericks has now incorporated, that significantly increases transfer speeds as opposed to AFP (apple file protocol). Switching the server to AFP from SMB2 DID cause the Firewire drive to spin down.
For those of you running Server app, obviously this is not a desirable solution as it will significantly impact the speed of your server. Apple Enterprise suggested shutting down your system as opposed to sleeping the Mac. Shutting down the Mac caused BOTH drives to spin down within about 10 seconds. Apple actually recommends that you shut down as opposed to sleep, which can interefere with Server functions. Apple themselves shut down ALL systems for all of their support centers and power them up 30 minutes prior to work. Apple has also directly stated that shutting down routinely is very energy effecient and completely safe (I don't think they would do it for all of their computers if it wasn't 100% safe).
For those of you running a simple client without Server, who are essentially using the drive for Time Machine or the like, the easiest solution is probably to connect the drive via USB or purchase an Apple Thunderbolt to Firewire adapter, which essentially treates the Firewire drive like a Thunderbolt one and will cause it to spin down on SMB2. This is another 30 bucks down the drain, but will solve the problem and maintain the speed of Firewire.
The new Mac Pros do not have Firewire, and because this issue does not occur with either USB3 or Thunderbolt (I was previously wrong), I do not think Apple will be working on a fix (Apple Enterprise indicated to me that this is NOT a known issue or problem).
Anyways - hope this post was helpful!