I have a few hard drives in this mac pro tower 10.9 Mavericks - they show up in Disk Utility but will not mount. DU shows a sort-of "blank document" or "unidentified filetype" image instead of the normal icon for external media. I used CCC to migrate data from a 3TB drive to a 4TB drive on a computer running a 10.8 Mountain Lion - and then (with the hard drives disconnected) upgraded that machine to 10.9 Mavericks. Since re-connecting, they will not mount. However, they will also no longer mount if I place them in a NAS connected to my MacBook Pro running 10.6 Snow Leopard. Both yes, are over 2.2 TB in size. They used to work before (and connected to each other via shared folders, etc, etc.