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I was trying to copy an iso of the windows installer onto a hard drive to avoid burning a disk. I first tried Disk Utility's restore function, however it didn't like the ISO for some reason. Then I tried using dd:

dd if=/path/to/image.iso of=/dev/disk3

I realized it was copying the file at a snail's pace, about 160 KB/sec. I rebooted into my linux installation and ran the command again, almost verbatim:

dd if=/path/to/image.iso of=/dev/sdc

This time the command executed in under a minute, with an average speed of 57 MB/sec. In both cases the source and destination were the same physical hard drives. What's going on?

I am running OSX 10.7.3 and Linux 2.6.38-13.

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Hmm, I would expect the linux one to go just as slow without a bs parameter. Do you have an alias for dd setup on linux (type alias at a prompt)? – Paul May 8 '12 at 1:30

1 Answer 1

up vote 16 down vote accepted

For OS X, use /dev/rdisk3.

For some reason rdisk is faster than disk. I believe it has to do with buffers.

Also in general using the bs flag with dd helps with speed.

dd if=/path/to/image.iso of=/dev/sdc bs=1M

The bytesize is 1M which transfers faster. On OS X you have to use 1m (lowercase) instead of 1M.

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Thank you ! I had forgotten to add bs=1m and it was slow as hell ! – LoremIpsum Oct 17 '14 at 6:35
The comment about the lowercase m on OS X was a lifesaver. Thank you very much! – macmadness86 Feb 28 at 8:52

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