You'll probably need to connect it via firewire again, to get the computer up and running with the new disk recognized.
Then bring up diskpart by going to start... run... and typing in diskpart. It should open a command line window.
Type "list disk" to show the disks installed on your system.
Type "select disk #" ie: "select disk 2" to select the new disk. Be sure you get this right. Selecting the wrong disk can mess up your system.
Type "list partition" to show the partitions on that disk.
Type "select partition #" to select the correct partition on this disk. I'm guessing you only have one partition so you'll type "select partition 1"
Type "detail partition" to see if it is active. It should show active and say yes or no near the top of the output.
If it is active, type "inactive"
****WARNING**** If you do this on your main hard drive, instead of the external drive, your system will not boot. BE SURE YOU SELECT THE CORRECT DRIVE
Edit to ADD: You can probably do this with the Disk Management GUI too. But, being mostly a Linux guy I'm more of a command line person and I'm not sure how to do it with Disk Management. Maybe someone else here can point you in the right direction if you'd rather do it that way.