On occasion FireWire buses can hang and stop responding. Should you run into this issue, here's are the suggested steps to resolve it.
If you have a hard drive freeze your FireWire bus and hang your machine, you can cause the system to reset the bus by plugging in a second device in the chain. The Mac will immediately rescan the bus and this will sometimes unfreeze the bus. If these steps fail to unfreeze the FireWire bus you will need to shut the machine down and restart the computer. You can resume your drive acquisition in MacForensicsLab after unfreezing the bus by checking the "Resume a previous recover." box under the Acquire function and selecting the previous image when prompted.