HDD available capacity : a beginner question for 1TB drives RAID 1

Discussion in 'Mac Pro' started by MCHR, Oct 2, 2010.

  1. MCHR macrumors regular

    Mar 13, 2009
    After doing a lengthy search (and finding some relevant information), I wanted to ask a specific question about final volume capacity in larger sized drives.

    Now that I understand the reason between the 'advertised' size and what we see on our Disc Utilities, I now know that a 1TB drive shows up as a 931GB volume. While I don't want to give away that 69GB capacity, I guess I'm good with that.

    My specific question is that I've just installed two WD RE3 1TB drives as a RAID 1 in a Synology NAS. Install went very easily. What I am now seeing as an available capacity is 912GB. A full 19GB less than the 931. I also understand that the OS creates folders, and future room for updates. But 19GB seems like a lot. At least to me.

    The Synology OS needed 300MB of room, and shows in the DSM panel.

    So, my elementary question is. . . . . does this seem correct? I have heard that larger capacity drives end up less than 93% of their advertised capacity. If anyone has any pointers or information, I'd appreciate the education.

    Otherwise, it seems like something may be wrong with the firmware etc in the drives themselves. Thanks for any info.
  2. Transporteur macrumors 68030

    Nov 30, 2008
    I'm not sure about your Synology NAS, but just out of curiosity, which OS are you using that your hard drives still show their "real" size instead of the advertised?
  3. MCHR thread starter macrumors regular

    Mar 13, 2009

    Those figures were shown in the Synology DSM (Disc Station Manager). Where the volume shows 931GB, and the available capacity shows 912GB. It also shows the space used for the DSM software is 299MB.

    So, in my (uneducated) mind that's a lot of un-explained capacity.
  4. Hellhammer Moderator


    Staff Member

    Dec 10, 2008
    What version of OS X are you running? In Leopard, 1GB is 1024MB while in Snow Leopard it's 1000MB so in Leopard, drives seem to have smaller capacity (it's the same but the way it's displayed is different).


    Two 1TB HDs should be 931GB but maybe the Synology box is taking 19GB for some reason. You could contact them and ask
  5. MCHR thread starter macrumors regular

    Mar 13, 2009
    That was what I thought. I've also posted on the Synology forum to see what others are seeing in real world installs.

    The Synology help desk indicates that 19GB is excessive and are looking into it. I've also contacted WD to see if formatting the RE drives uses more room, though I doubt it.
  6. alphaod macrumors Core


    Feb 9, 2008
    Manufacturers like to say 1TB = 1,000,000,000,000 bytes which is base 10. In reality 1TB = 1,099,511,627,776 bytes because computers are in binary (base 2); so if you do the conversion backwards from 1,000,000,000,000 bytes back, it should be 0.909494702TB or 931.322575GB.

    So you didn't lose any capacity; manufacturers use base 10 to make their drives looks big while computers read and calculate it in binary and do the conversion that way which nets a "smaller" number.

    Also Apple changed Snow Leopard (10.6) to display the figures in base 10 to match what the manufacturers are doing.

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