So I just purchased a HDD upgrade kit today and installed a second 500 gig 5400 rpm HDD I was using in an external USB case. I formatted the drive yesterday and installed it today. If I try and write to the drive I get "The operation canít be completed because an unexpected error occurred (error code -50)." Under the disc "Get info" permissions are Read & Write for the admin, Read & Write for staff and Read only for everyone. In Disc Utility I can verify and repair the disc but I cannot erase or partition it. It says that it cannot unmount the disc.
So I pulled the drive out and hooked it up to my wife's macbook via usb and it worked fine. I reformatted it and gave it a different name but still have the same "The operation canít be completed because an unexpected error occurred (error code -50)." message.
Finally I tried another hard drive, seagate 320 gig 5400 rpm and it has the same problem. I tried formatting the 500 gig in a ps3 so OSX would need to force initialize it and that did not work. Both drives work fine and have no issues when attached to my seagate thunderbolt adapter, it is only when they are hooked up internally.

Any one else have experience with a second internal HDD or this particular problem? Any help is appreciated in advance.