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camner 10-26-2013 02:54 PM

SD compressed images vs Disk Utility "convert" option
When SD is asked to create a read-only image, it is a two-step process. SD first makes a sparseimage, and then creates a second file with dmg extension which is the final, desired file.

There are times when I wish to save time by just making a sparseimage with SD and then at a later time want to save the space on the destination volume by creating a compressed dmg file.

There are two ways I can think of to do this:
  1. Mount the sparseimage, then use SD just as I would have from the original source. This will be a two step process again of creating a new sparseimage and then a dmg file
  2. Mount the sparseimage and then use Disk Utility's convert option to convert the sparseimage

I've done both, and when I tried #2 I noticed that Disk Utility did NOT create the intermediate sparseimage file. So, are these two processes different? Is one preferable to the other, either in efficiency (time) or some other factor?

dnanian 10-26-2013 02:58 PM

They're unmounting the drive and doing a low-level, sector copy. The way we do it is the only way to do it file by file - clearly, you can't write into a read-only image...

camner 10-26-2013 03:10 PM

Is this a distinction without an important difference? (That is, the end result is the same and the methodology difference shouldn't matter to end user?)

dnanian 10-26-2013 03:11 PM

No, it matters. You can't copy selectively (as you have to) unmounted, and you can't unmount a volume you're using (such as one you're booted from).

The Image option in the advanced tab of Options does the low-level copy, but it requires the volume to be unmountable (which the backup drive should be, unless something is using it).

camner 10-26-2013 05:04 PM

Color me potentially confused...sorry if I'm being dense.

1. It seems to me that if I want to clone a physical volume to a physical volume AND I wish to have a compressed, read-only disk image of the source volume, then I go to the SD Options-->Advanced tab and click the "Image" box, yes?

2. If I want to end up with ONLY a compressed, read-only disk image of the source (and I don't need a clone on a physical volume), then I just need to select "Disk image" in the target drop down on the main SD window.


As far as Disk Utility "Convert" vs SD, I understand that Disk Utility will do a block by block copy whereas SD will do a file by file copy (at least I think I understand this).

BUT, why do I as an end user care which method (block by block vs file by file) gets used? Don't I end up with a compressed, read-only DMG file either way, and the two DMG files are identical in functionality, right?

Again, thanks for your responsiveness and sorry I'm being stupid here.

dnanian 10-26-2013 05:06 PM

Yes to both.

Again, because we can be selective during copy for 1), but not for 2), and since 1) has been selective during the copy to the destination, then you can make a quick image of it.

Also, again, you can't copy direct unless you can unmount the volume, which you can't do for your boot drive, or any drive you're using.

camner 10-26-2013 06:29 PM

Thanks much. I think I understand now.

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