Unable to use firewire cable to transfer data between two Macs

Both my iMac and my MacBookPro are less than a year old. I have transferred data from one to the other before using firewire cable 4. Now, suddenly, I cannot get it to work. I followed all the instructions. Assume for this example that I want to transfer datafrom the iMac to the laptop. I disconnected all firewire cables on both computers. I turned the iMac on. I turned the laptop off. I connected the two computers using the firewire cable. I turned on the laptop while holding the T key down. The laptop screen displays a Y. An icon appears on my iMac desktop, ostensibly representing the hard drive on the laptop. But when I click on that icon, the drive I get is the same as the hard drive on my iMac!