Was the external drive originally formatted on the Mac or PC? If on the PC, what did you format it as (FAT32, exFAT, NTFS)? Have you checked the status of the drive on the PC in Disk Management?
It was originally formatted for PC but I believe I changed it before I started using it on my Mac , but cant remember what to. I have been only using it on my ps3 and my mac. It shows on Device Manager for a PC but won't show in Disk Management on the PC. It wont show in Command Prompt or the Mac equivalent either.