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I have DMG I need to mount and write a file to. When I mount it, finder shows the root and all as read-only.

Then I discovered 'mount -w'. But when I run that, I get 'mount: unknown special file or file system.'

How can I mount this disk image as writable, or force a file into it?

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4 Answers 4

up vote 7 down vote accepted

Almost all read-only DMGs are actually compressed (UDZ0 -- see the hdiutil(1) man page), so their file format doesn't support simply "flipping a bit" to make them writable.

You could use Disk Utility or

hdiutil imageinfo filename.dmg  

...to see what format your disk image is in. Then you could use Disk Utility or something like

hdiutil convert filename.dmg -format UDSP -o filename.sparseimage  

...to convert it to a read-write format. Note that the conversion does not happen in-place, so you'll have to tell it to put the output file on a filesystem that has enough room for an uncompressed copy of all the data from your .dmg.

The .dmg extension does not guarantee that the image is actually compressed, but that's by far the most likely possibility.

There are several other possibilities available to you with hdiutil. For instance, if your .dmg is actually uncompressed read-only (UDRO), it might be possible to force it to mount read-write. Also, if you want to leave your .dmg compressed but still want to mount it in a writable fashion, you can mount it with a "shadow file"; all writes actually get written to the shadow file.

Update: Other Answers on this Question seem to think .dmg always means UDZ0 which just isn't true. From the hdiutil(1) man page, here are the list of internal formats a .dmg can have (note that a couple of these can have different default filename extensions like .sparseimage, but I'm pretty sure that's not a hard-and-fast rule either).

UDRW - UDIF read/write image  
UDRO - UDIF read-only image  
UDCO - UDIF ADC-compressed image  
UDZO - UDIF zlib-compressed image  
UDBZ - UDIF bzip2-compressed image (OS X 10.4+ only)  
UFBI - UDIF entire image with MD5 checksum  
UDRo - UDIF read-only (obsolete format)  
UDCo - UDIF compressed (obsolete format)  
UDTO - DVD/CD-R master for export  
UDxx - UDIF stub image  
UDSP - SPARSE (grows with content)  
UDSB - SPARSEBUNDLE (grows with content; bundle-backed)  
RdWr - NDIF read/write image (deprecated)  
Rdxx - NDIF read-only image (Disk Copy 6.3.3 format)  
ROCo - NDIF compressed image (deprecated)  
Rken - NDIF compressed (obsolete format)  
DC42 - Disk Copy 4.2 image  
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I didn't assume UDZ0, but all of the types except UDRW, and UDSP (maybe UDxx UDSB or UDT0) are read only. I assumed that all dmgs were compressed with SOMETHING, and so not writable... I've never knowingly come across a NON-compressed DMG... –  Brian Postow May 27 '10 at 18:51
    
@Brian - I use an encrypted UDSP routinely for storing sensitive information on my laptop (passwords, bank info, etc). –  KeithB May 27 '10 at 19:57
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if there's enough "space" on the Rd-onry DMG, you can convert to a R/W DMG using Disk Utility, and convert it back to a compressed DMG if you want. Otherwise, you can "expand" the DMG using hdiutil

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You can't write to a DMG. it makes no sense. You need to copy everything out, edit, and make a new DMG.

Sorry.

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That's just not true. You can create read-write .dmg files. Also, you don't need to copy everything out, you can simply convert the disk image to a writable format. See my Answer. –  Spiff May 27 '10 at 18:28
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@Spiff, I suppose, but your solution is EFFECTIVELY the the same as mine, as it has to de-compress, the you add the new file, and re-compress. If it's not in-place, then you're not REALLY writing to the existing DMG... –  Brian Postow May 27 '10 at 18:47
    
Your answer contains false information. –  brian Feb 10 '11 at 2:53
    
-1 but if you change the line "You can't write to a DMG" to something that is actually true, like, "You can't write to a readonly DMG" I'll change my vote. Thanks! –  Yar Feb 17 '12 at 3:05
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A regular DMG image is not writable as there's no way to add space to it. The solution as noted by @Brian_Postow is to copy out the entire contents and create a whole new DMG file. If you plan to do this a lot, next time create a sparse image as these are writable.

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That's not exactly true. The .dmg extension does not necessarily mean that it's read-only. See my Answer. –  Spiff May 27 '10 at 18:20
    
I should have said "default" instead of "regular". Good info on converting them though. –  Chris Nava May 28 '10 at 4:07
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