It appears you bought an 'unformatted' (non-provided manufacturer and model information) hard disk drive, hdd, connected it to your PowerBook G4 (running an non-provided version of MacOS X) and have not yet 'formatted' it.
Launch (open, run) 'Disk Utility', locate and click on the external hdd's icon - in the left side column, click on the 'Erase' tab, select a 'Volume Format:' ('Mac OS Extended' or 'Mac OS Extended (Journaled)', enter a desired 'Name:', (optionally, leave the 'Install mac OS 9 Disk Driver' check marked), click the 'Erase' button, and any further window's buttons to complete the erase process.
Only after the above process is completed will an icon representing the external hdd, or icons representing mulitple partitions (if you partitioned the hdd via 'Disk Utility's 'Partition' tab's panels of options), appear on your 'Desktop' and then be accessible by you.