How to create a DMG file I can add files to in the future (encrypted)

[KES3059A3]

I'm using Disk Utility to try to figure out how to create a DMG file that is encrypted, and you need a password to open. I want this DMG file to be able to have items added to it (and deleted from it) in the future without needed to create a new disk image (so I want the DMG to esentailly act like a storage device--I think that's "read/write", right?).

For the life of me, I can't figure out how to do this in macOS 26's Disk Utility.

For the best security, what Format do I use, as well as what Encryption, Partitions, and Image Format?

 

[Fishrrman]

There doesn't seem to be anything hard about this.
Just open disk utility, and select the options you want:

The above shows [approximately] how I would set it up.

The main question is:
What SIZE do you need it to be?
You don't want "too small" (although if it ever filled up, you could create a new, larger image, move things over, then delete the old one).
But you don't want it too LARGE, either.
Be aware that you can't change the size, once you've created it.

I prefer HFS+ (Mac OS extended, journaled, single partition, GUID partition map).
You could use APFS if you wanted to, but HFS+ "just works" for me.

Select the level of encryption you want and enter the desired password.
YOU MUST REMEMBER THIS PASSWORD, so write it down on paper and hide it if need be.

Then, give it a meaningful name and save it where you want.

Not much more to it than that.
Try creating one.
If you don't like it, just delete it and try again.

 

[KES3059A3]

There doesn't seem to be anything hard about this.
Just open disk utility, and select the options you want:

View attachment 2567269
The above shows [approximately] how I would set it up.

The main question is:
What SIZE do you need it to be?
You don't want "too small" (although if it ever filled up, you could create a new, larger image, move things over, then delete the old one).
But you don't want it too LARGE, either.
Be aware that you can't change the size, once you've created it.

I prefer HFS+ (Mac OS extended, journaled, single partition, GUID partition map).
You could use APFS if you wanted to, but HFS+ "just works" for me.

Select the level of encryption you want and enter the desired password.
YOU MUST REMEMBER THIS PASSWORD, so write it down on paper and hide it if need be.

Then, give it a meaningful name and save it where you want.

Not much more to it than that.
Try creating one.
If you don't like it, just delete it and try again.

Thanks for your reply. But from your screenshot, it looks like you aren't using macOS 26 yet?

In macOS 26, the option to select "read/write disk image" from the Image Format menu has been removed. That's why I can't figure out how to create a read/write DMG now.

 

[bob_zz123]

I would use a Sparse Bundle or the brand new ASIF sparse format which you can create in Disk Utility (with Encryption set to 256 bit and format APFS assuming it will only be used on Apple devices). The advantage of these formats is that you can designate a maximum size but unlike the old read/write disk image, the actual size on disk of the image grows (up to the maximum size) as opposed to all being allocated in one go.

Example - 10GB disk image - in old read/write format - it would be 10GB on disk even if you only had 1GB of files stored in it.

In sparse bundle or ASIF format - it would only consume 1GB on disk if you had 1GB of files on it and would only consume the full 10GB when you had filled the bundle full of 10GB of files.

Both these formats are read/write and you can learn more about the new ASIF format here: https://eclecticlight.co/2025/06/12/macos-tahoe-brings-a-new-disk-image-format/

 

[Fishrrman]

"In macOS 26, the option to select "read/write disk image" from the Image Format menu has been removed. That's why I can't figure out how to create a read/write DMG now."

Have you TRIED creating one, to see how it works out?
Doesn't make sense to me that they would make them all "read only".
Again, just TRY it once, and see what happens.

Speaking ONLY for myself, I wouldn't touch a "sparse bundle" with a ten-foot pole...

 

[KES3059A3]

"In macOS 26, the option to select "read/write disk image" from the Image Format menu has been removed. That's why I can't figure out how to create a read/write DMG now."

Have you TRIED creating one, to see how it works out?
Doesn't make sense to me that they would make them all "read only".
Again, just TRY it once, and see what happens.

Of course I tried. That's why I posted here asking how you now do it in macOS 26.

 

[KES3059A3]

I would use a Sparse Bundle or the brand new ASIF sparse format which you can create in Disk Utility (with Encryption set to 256 bit and format APFS assuming it will only be used on Apple devices). The advantage of these formats is that you can designate a maximum size but unlike the old read/write disk image, the actual size on disk of the image grows (up to the maximum size) as opposed to all being allocated in one go.

Example - 10GB disk image - in old read/write format - it would be 10GB on disk even if you only had 1GB of files stored in it.

In sparse bundle or ASIF format - it would only consume 1GB on disk if you had 1GB of files on it and would only consume the full 10GB when you had filled the bundle full of 10GB of files.

Both these formats are read/write and you can learn more about the new ASIF format here: https://eclecticlight.co/2025/06/12/macos-tahoe-brings-a-new-disk-image-format/

Thank you!

 

[Fishrrman]

"That's why I posted here asking how you now do it in macOS 26."

I booted up from my external drive containing my test copy of Tahoe.

I opened disk utility.

From the file menu, I choose “new image —> blank image”.

I chose “documents” as the destination.

I set it up as follows:

When I clicked “save” it asked me to enter (and then re-enter) a password.

I did.

The disk image was created successfully.

It mounts with the password, and I can read AND write to it (as set up above).
-----------------------
Have you tried this?

One thing I did find out.
I kept the size of the image small (20mb).

When I opened a test file I had saved onto it, and made a change, and then saved it, the OS warned me that it would only save the current (saved) version, without any prior versions (or something like that). That was because the image was too small to include previous "versions"... again, something like that. Make the image larger and it probably won't happen (I don't know HOW large an image has to be before it includes previous versions...)

Don't know what would happen with a smaller-sized sparse bundle image.

 

[bogdanw]

Explained in the manual https://support.apple.com/guide/disk-utility/create-a-disk-image-dskutl11888/mac
"Create a blank disk image for storage
...
Click the Image Format pop-up menu, then choose an option:
* Sparse Bundle Image (UDSB): A read/write image that grows with the data stored in it. The data is stored in multiple files. This structure is often used for Time Machine backups over network.
* RAW Image: An image that allows you to add, remove, or edit files in the disk image after itʼs created. The data is stored in the image file exactly as it is laid down in the virtual disk represented by the image.
* Apple Sparse Image (ASIF): A modern sparse read/write image. The space it uses on your disk is proportional to the data it contains, making it efficient and general-purpose.
* DVD/CD master: An image format that changes the size of the image to 177 MB (CD 8 cm). The format uses the .cdr file extension."