Ok try this:
First try to see if the Windows Disk Manager recognizes the unit in W Vista/ 7 click on start then second button over Computer then administrate
Then format from there, if the disk doesnt format even there then dowload the Low Level Format Tool Here http://hddguru.com/software/HDD-LLF-Low-Level-Format-Tool/HDDLLF.4.25.exe
and the HP format Tool herehttp://files.extremeoverclocking.com/file.php?f=197
then oppen the Low Level Format tool and select the disk from the list, click continue and then select Low Level Format, uncheck the fast wipe option and click the format this drive button.
The procces will take some time my 320gb drive taked 3 hours.
When the procces finishes close the program and go to the HP format tool, again select your hard disk from the list and then format it as NTFS (the fast format option must remain uncheck) it will take some more time aprox 40 min to 1 hour but with that it should work, that hapenn to my 320 gb Adata external disk and now it works like a charm. if this doesnt help and the disk has never been droppend then is most likely an electrical damage in the disk motherboard-pcb.
Im not an expert in this but that's what i made and worked perfectly i hope it works to you.
Cheers and excuse my bad english.