Launch 'System Profiler' ('/Applications/Utilities/'), click on the 'Hardware' disclosure triangle (to make it point down), and then click on 'ATA'.
In the panel titled 'ATA Device Tree' locate 'ATA Bus' (you may have to click on its disclosure triangle ...), and click on one of its listed items (which will be either the Combo or hard disk drive).
Beneath the 'ATA Bus' list is a description panel, which provides detailed information about the selected 'ATA Bus' item.
If you see 'Capacity:' listed along the left hand side - you have selected the hard disk drive.
Beneath 'Capacity:' is the 'Model:' identifier. You can select the text to its right. Do so, and copy it.
You now have the model number of your eMac's hard drive (in the clipboard). Go to
Google, paste the information into the search field, and click the 'Google Search' button. You will then be presented with links to information (manufacturer, speed, dimensions, etc) about your eMac's hard disk drive.