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I'm trying to resize the *.dmg of my MAC OS X installation file because I have only DVD of 4.7 GB and not dual layer disks,at the moment. The problem is that the dmg file is almost 7 GB large and I don't know how to reduce it to 4.7 GB. So,after some research I found a script in this web site :

http://hints.macworld.com/article.php?story=20030306095219454

I run it,but I don't know really if it worked,because it has resized the original file only by 1% ! as it seems. Check by yourself the log below...

marks-Mac:Desktop mark$ ./dmgcon.sh Macos.dmg
Preparing imaging engine…
Reading Protective Master Boot Record (MBR : 0)…
   (CRC32 $8E0A8B19: Protective Master Boot Record (MBR : 0))
Reading GPT Header (Primary GPT Header : 1)…
   (CRC32 $2D71B467: GPT Header (Primary GPT Header : 1))
Reading GPT Partition Data (Primary GPT Table : 2)…
   (CRC32 $75C8939C: GPT Partition Data (Primary GPT Table : 2))
Reading  (Apple_Free : 3)…
   (CRC32 $00000000:  (Apple_Free : 3))
Reading EFI System Partition (C12A7328-F81F-11D2-BA4B-00A0C93EC93B : 4)…
.
   (CRC32 $571DC111: EFI System Partition (C12A7328-F81F-11D2-BA4B-00A0C93EC93B : 4))
Reading Untitled 2 (Apple_HFS : 5)…
..............................................................................
   (CRC32 $CCE2CD3E: Untitled 2 (Apple_HFS : 5))
Reading  (Apple_Free : 6)…
..............................................................................
   (CRC32 $00000000:  (Apple_Free : 6))
Reading GPT Partition Data (Backup GPT Table : 7)…
...............................................................................
   (CRC32 $75C8939C: GPT Partition Data (Backup GPT Table : 7))
Reading GPT Header (Backup GPT Header : 8)…
...............................................................................
   (CRC32 $C035E623: GPT Header (Backup GPT Header : 8))
Adding resources…
...............................................................................
Elapsed Time:  5m  9.325s
File size: 6479801806 bytes, Checksum: CRC32 $1639F7E5
Sectors processed: 30556288, 12815548 compressed
Speed: 20.2Mbytes/sec
Savings: 58.6%
created: /Users/mark/Desktop/Macos.dmg.compressed.dmg
-rw-r--r--@ 1 mark  staff  6479801806 Dec  9 12:08 Macos.dmg
-rwxr-xr-x@ 1 mark  staff  6561578523 Oct 27 08:34 Macos.dmg.bak*

it says : savings 58.6% ? if the original file is now called Macos.dmg and its dimensions is : 6.561.578.523 bytes (6,58 GB),the compressed file is : 6.479.801.806 bytes (6,48 GB),the difference is only of 1% less...if everything is correct,this script is not useful at all...let me know please I have did some kind of mistake...thanks...

  • I'm not an expert of the mac world,so maybe I used a wrong tag... – Marietto Dec 9 '18 at 21:32
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You cannot compress this type of DMG for multiple reasons - so you won't have success with this scripts nor others like it. I'll suggest looking at alternative options, such as buying a dual-layer disk, or simply using a different storage medium such as a USB pen-drive or an external USB hard drive.

The reasons this type of DMG don't compress are for example:

1) Apple has already used compression on most relevant parts of the installer image - so there's not really much left to compress.

2) The disk image size is really irrelevant - the important is how much space it takes up on the DVD, and I'm afraid compressing the DMG won't change that. What you write onto the DVD is not a bit-for-bit copy of the DMG, but rather it would be uncompressed before writing onto the DMG. If not, it wouldn't be bootable nor useable for installing macOS.

  • ok. Can I remove something inside the dmg file that's not strictly needed and that I can install later in another way ? simply deleting files and folders ? – Marietto Dec 9 '18 at 23:58
  • You cannot simply delete any files/folders, nor do I know of anything obvious you can just delete and then install later. For older versions of macOS, some enthusiasists created smaller install images, but I think that trend stopped years ago when the DVD went out of style. Which version of macOS do you want to install? – jksoegaard Dec 10 '18 at 8:35
  • Hello. Where are you ? Do u know if there is a way to split the dmg file of a mac os mojave into two DVDs making the first one bootable and the second one contains the rest of the installation ? – Marietto Dec 10 '18 at 17:38
  • OR : another way could be to convert the DMG file in ISO and then split it in two files and boot from the first one... – Marietto Dec 10 '18 at 18:28
  • What do you mean - where are you? ..... And no, you cannot split the dmg or ISO into two and getting a working installer from that. Is there any particular reason why you haven't got a USB pendrive, or cannot buy one at the local super market? - It seems you're willing to go to a lot of trouble that would be completely unnecessary using such commonly available tools? – jksoegaard Dec 10 '18 at 23:24

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