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I have bought this hard drive: Seagate ST3000DM which is a 3TB 3.5" hard drive. When I connect it via a USB2 docking station, here is what OS X sees:

Disk Description :  ST3000DM 001-9YN166 Media   Total Capacity :    801.57 GB (801 569 726 464 Bytes)
Connection Bus :    USB Write Status :  Read/Write
Connection Type :   External    S.M.A.R.T. Status : Not Supported
USB Serial Number : (removed)   Partition Map Scheme :  Unformatted

Is there a software solution to this problem ? I have seen solutions that imply to buy a USB3 docking station but MacBookPro 7.1 only have USB2 ports.

Is there an hardware solution that don't imply buying a NAS, which is about two or three times the price of the disk itself ?

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It may be a problem with the docking station. Some USB enclosures (typically older ones) don't have support for drives using "Advanced Formatting" (which is used for drives >2TB). You might try getting a different USB enclosure (check reviews on Amazon or Newegg to be sure it works with large drives) and see if that solves the issue. It shouldn't be too expensive — $30 or so, and it's probably worth buying from somewhere you can return it easily if it doesn't do the trick. – robmathers Dec 6 '12 at 16:24
I have found this product SYBA CL-ENC50038, but I'm not sure if it will work correctly when I connect it on a USB2 port, if that's even possible. And I have already spent too much time on this hard drive problem.. – alecail Dec 6 '12 at 16:39
From the Newegg reviews, that looks like it should do fine with a 3TB drive, assuming the problem is your enclosure in the first place (which I suspect is it, but hard to tell for sure based on the current information). USB 3 is fully backwards compatible with USB 2, so you shouldn't have any problems connecting it to your MacBook. – robmathers Dec 6 '12 at 16:42

I'm concerned that the Partition Map Scheme shows up as "Unformatted".

I suggest you run Disk Utility and format the drive with Mac OSX Extended (Journaled) format to see what happens to the Total Capacity and to set the PM Scheme (which should be GUID).

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Check with seagate for firmware for Apple products, I have one Seagate drive that needed the firmware. Sadly to say but but I have not had very good luck using this with time machine. I need another call to Segate, by the way they have beeb very helpful in the past.

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