I'm having problems using an external 3TB hard drive. I've tried using both MBR and GPT, and in both cases my computer only sees 746GB. However, I have another 3TB hard drive (also Western Digital, but not identical) which works perfectly fine in the same enclosure.

I should also point out that my computer is a bit old, and only supports 2.2TB hard drives when connected internally via SATA. However, as far as I know, this issue should not exist with USB.

So my question is, why does one hard drive work fine, and the other only shows 746GB in the same USB enclosure? It must be a difference between the hard drives, but what exactly, and how do I find an enclosure that supports "it"?

  • I have this same problem on the Banana Pi. I know what I am doing too (it isn't MBR). The harddrive is formatted GUID with one partition filling the drive formatted to ext4. My Ubuntu computer sees it fine (in USB chassis). The Banana Pi does not see the partition, just shows another device with 746gb capacity. I wish I could solve this... – Jonathan Komar Mar 14 '15 at 19:50

The difference between drives is sector size: 512 bytes vs 4KB.

There is an article explaining it in Wikipedia and on Western Digital's site.

The product must be new (2011?)

  • how does this cause the size difference wierdness tho? – Journeyman Geek May 28 '12 at 2:50
  • 4
    It most likely is caused by binary sector count showing only a fraction of integer division by 2.2Tb. Divide 3Tb by 2.2Tb you get 1 integer count and remainder of some 7XXGb. The integer part is lost after truncation, so the OS sees only 7XXGb – user27803 May 28 '12 at 2:56
  • @user27803: I'm afraid your original answer is a highly unlikely explanation. Although modern hard drives use 4K blocks internally, almost all of them present 512-byte blocks at the interface. Drives that work in terms of 4K blocks at the interface are extremely uncommon (even here in 2015), and are presently limited to "enterprise-class" drives. If the drive doesn't have a "4Kn" logo on it is NOT one of those drives. Drives that use 4K blocks internally but "speak" in terms of 512-byte blocks at the interface should have an "AF" logo. – Jamie Hanrahan Mar 6 '15 at 20:40
  • Please note that if the drive was reporting its capacity in terms of 4k sectors, but the OS thought the number represented 512-byte sectors, the reported capacity would not be 7xx GB. It would be about 375 GB. (3 GB / 4096 = about 730 million sectors; interpreting these as 512-byte sectors yields 375 GB.) – Jamie Hanrahan Mar 6 '15 at 20:46

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