Q: How can I change Time Machine Preferences so that computer will recognize my back up disk?

Hello sberman.  Thanks for your reply.  I appreciate it but it did not really help.  I can click on System Preferences (from the Dock) and then I can choose Time Machine from the icons.  But when I click on "Select Disks" (it's actually "Select Backup Disk, I am led to a window which has only one option, namely: "Other Airport Time Capsule."  I don't have my Seagate external hard drive as one of the available options.  How do I add the Seagate External Hard drive to the list of available disks?  Any help you can provide (or that another member of the community can provide) is greatly appreciated.