I recently purchased an official 320GB HD for my 360 Slim, but I seem to be having an issue with it. When I install the drive into the storage compartment (making sure it's seated all the way in), it doesn't show up in my list of Storage Devices when I start up my 360. I've tried the troubleshooting steps listed in the lit that came with the hard drive, but so far, nothing has worked. No amount of reboots, cache cleaning, or installing the latest update from USB, etc., has worked.
I borrowed another hard drive from a friend that shows up fine, so I assumed it was just the one I purchased. I returned it to the store I bought it from (Game Stop), and exchanged it for another new, unopened official Microsoft HD. Unfortunately, this HD also had the same results as the first one.
The drive itself appears to be fully functional, as I was able to install it into an external SATA adapter and could both format it on my PC, as well as use it as USB storage on the Xbox. However, it still will not show up in my Storage Device list when correctly inserted into the drive slot.
I'm at a loss on what to try next. Using it as USB storage is not an option, as it only allows 32GB to be used out of 320GB. Plus, some games I have will only run/install on an actual HD, not on USB or flash storage.