Model GPHDUC

Welcome to Tom's Hardware Forums,
A good place to start will be to locate the model number of the computer itself (not the external hardware such as the Iomega external hard drive) which can be found either on the back or bottom of the computer. From that, you should be able to find the appropriate support (user manual) from apple here http://www.apple.com/support/
If you are looking for a manual for the Iomega external hard drive, I believe this may be your product https://iomega-na-en.custhelp.com/app/cust_alp/p/3,1130/c/73 - the manual is available there (it may be a different product, you will want to confirm that)
Hope it helps