and the reverse too. What I'm seeing is that when I format a HDD internally as GUID Partition Table / MacOS Extended Journalled then everything appears to be fine. If I then move the disk to an external USB 3.0 interface (both Seagate Goflex and Hitachi Tuoro exhibit the same problem), then the disk isn't recognised and needs to be reformatted. If i do the reverse i.e. reformat externally then everything is fine until I move the disk internally, where it's is no longer recognised. I'm running OS X 10.9.5 (13F1077) on a Mac Pro 2008 The discs showing the problem are either Seagate or Hitachi, all greater than 2TB. I haven't tried smaller as I don't have easy access right now. The external interfaces are from external discs that originally had discs greater than 2TB I'm using a USB 3.0 PCIE card (had this for around a year), though I think I've seen the same problem with a Firewire 800 interface - maybe need to check this. I dont recall having this problem before, but since I don't move disks around that often i'm not sure what if any changes I've made have caused this. I'm stumped here -is there something I'm missing?