New disk image format for macOS 15 and 26 goes into turbo mode
Encrypted disk images are notoriously slow on the Mac. The new ASIF format aims to change this. It is available for Tahoe and Sequoia.
External SSD: If you want to be on the safe side, encrypt such media.
(Image: c't)
A practical detail in the upcoming new Mac operating system macOS 26 aka Tahoe: Apple has tweaked its encrypted disk images. As IT blogger and Mac & i author Howard Oakley has noticed, the Mac operating system has been given a new format that should be significantly more performant than the previous sparse image format. According to the documentation now available, almost native speeds are possible. This makes encrypted disk images much more suitable for everyday use.
Creation only with Tahoe
With a disk image, it is possible to create an entire drive on an external or internal medium that can be mounted and then used as if it were a "real" SSD, for example. However, Apple's standard format for an encrypted sparse image only packs around 100 MBytes per second, even if the SSD on which it is stored is significantly faster. The new format is called ASIF. It appears to have been developed in connection with improvements to virtualization, including a new container function for macOS.
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Currently, it is only possible to write ASIF disk images under macOS 26. This is done either in the terminal via the diskutil command or via the disk utility. macOS 15 alias Sequoia cannot yet generate ASIF disk images even in the latest version 15.5, and hdiutil is not yet running either. However, mounting and reading/writing to ASIFs already appears to be possible in Sequoia. It remains to be seen whether Apple will retrofit creation with macOS 15.6; it may become an exclusive feature of macOS 26.
Bye-bye sparse bundles?
Previously, it had always been recommended to use so-called sparse bundles instead of sparse images, as these were always more performant. Oakley suspects that this could now change with ASIF. This also offers various advantages, such as the fact that images are a complete file. Disk images should therefore gradually switch to ASIF with Tahoe. The DropDMG tool, which makes handling much easier, should be equipped with this soon.
The new operating system will probably be released in September. If you would like to try out ASIF, you still have to use the developer beta of macOS 26. A public beta will be released in July.
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