It could also be the Blue Light of Death. I had something similar a while ago but I can't really remember all of the details. The recommended solution was to download a file off of Sony's website to fix it. Ultimately that did not fix my issue completely and I had to send it in. So my main answer would be, if it's still under warranty, check that first.
I'm not an expert, but try a different usb cable and see if that improves the speed. Sometimes it's the simplest things that give the most logical solutions. Other than that, these stupid external hard drives are typically soldered in a format that can only be accessed by usb. You'd think that once you open up these units, you would see a bare hard drive and an SATA to USB Adapter but unfortunately they screw you and allow you no way to access it via SATA. I don't understand that you said you can access your files but then you can't.. You might want to try downloading a hdd checking tool like HDtune or something and see if your hdd is malfunctioning.