gmirror, gpart and MBR vs GPT in the Handbook

Warren Block wblock at
Mon Dec 2 21:21:51 UTC 2013

On Mon, 2 Dec 2013, Mike. wrote:

> My understanding is that MBR can be used with drives only up to
> and including 2TB in size.  So if I use MBR, the maximum drive
> size I could use would be 2TB.  Is that correct?

As far as I recall, MBR expects 512-byte blocks and uses 32-bit values. 
The total number works out to 2TB (2^32*512).

Just recently, someone in the forums had an external drive, MBR format, 
but 4K blocks.  There were problems, although I don't know if FreeBSD 
was at fault.

I have not tried MBR on a drive larger than 2TB.  I suspect it would be 
safe to use the first 2TB and ignore the rest, but that really ought to 
be verified.

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