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In Disk Utility, one can create disk images from arbitrary folders; even encrypting them if you like. However, there doesn't seem to be a way of setting the image size, so once you run out of space, you need to start over. Disk Utility has a "Resize Image" option, but it appears to be quite limited (I can't get it to work on rw images).

So far, the only procedure I've found to create a disk image of an arbitrary size is:

  • Create a big, empty file in your otherwise clean directory:

    dd if=/dev/zero of=zeros bs=1024M count=5
    
  • Create a disk image of that folder.

  • Mount the image, erase its contents (i.e., the zeros file) and copy the actual data you want in there.

Is there a simpler way?

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    I'm sure this question must exist here somewhere, but my google-fu is failing me right now. The trick is to use a .sparsebundle, which will auto-expand/contract - see cnet.com/tech/computing/…
    – Tetsujin
    Sep 10 at 15:30
  • Both regular sparse images and sparse bundles can expand as you add files to them. There's a maximum size you set when creating them, but you can set that to something huge. Sep 10 at 22:48
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Often using the Graphic User Interface (GUI) offered by the Disk Utility application is more convenient than using a Command Line Interface (CLI) required by macOS commands. However, the Disk Utility application only offers a limited subset of the capabilities of various macOS commands. In your case, to resize an image you would need to use the hdiutil command. Below is an example of the syntax.

hdiutil resize -size size_spec image 

The size specifiers (size_spec) can be the following.

-size ??b|??k|??m|??g|??t|??p|??e

Here ?? needs to be replaced by a number. The letters represent the following multipliers.

b is bytes (not blocks) where the multiplier is 1.

k is power of two kibibytes where the multiplier is 1024 (1 x 2^10).

m is power of two mebibytes where the multiplier is 1048576 (1 x 2^20).

g is power of two gibibytes where the multiplier is 1073741824 (1 x 2^30).

t is power of two tebibytes where the multiplier is 1099511627776 (1 x 2^40).

p is power of two pebibytes where the multiplier is 1125899906842624 (1 x 2^50).

e is power of two exbibytes where the multiplier is 1152921504606846976 (1 x 2^60).

or

-sectors sector_count | min

Specify the number of 512-byte sectors to which the partition should be resized. If this falls outside the minimum valid value or space remaining on the underlying file system, an error will be returned and the partition will not be resized. min automatically determines the smallest possible size.

For example, the following command could be applied to an ejected image.

hdiutil resize -size 3m sample.dmg

Afterwards, the command gpt -r show sample.dmg could be used to verify the result. In this case, the command would produce the following output.

  start   size  index  contents
      0      1         PMBR
      1      1         Pri GPT header
      2     32         Pri GPT table
     34      6         
     40   6144      1  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
   6184      3         
   6187     32         Sec GPT table
   6219      1         Sec GPT header

Where the partition holding the volume is shown as exactly 6144 sectors = 3 x 2^20 bytes / 512 bytes per sector.

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