Following the recent upgrade of AirPort Extreme to 7.3.1 I attempted to mount my connected Seagate FreeAgent Pro drive. I discovered a few things:
1) The FreeAgent Drives cannot be formatted for Mac by Leopard Disk Utility. I used a Tiger machine to format the drive as Mac Extended with journalling and it was recognized by the AirPort Extreme.
2) Once the drive is recognized by the AirPort Utility you MUST connect to the drive (the default is your AirPort Extreme password) and you MUST copy at least one file to the drive.
3) Once a file is copied to the drive, it appears in Time Machine and is ready for wireless backup.