Unlike many other file formats, the ISO file extension does not use compression, as it’s a full digital copy of the original disc. This information stored on the ISO file can be “mounted” and used on your computer without a CD player, be opened to explore the contents, or simply burned onto a physical disc of your choice.

The ISO file extension does not just store files and folders they house all the vital file system information about the disc’s structure. Select the volume that you want to start up from.Ĭlick Continue and the Mac restarts from that volume.An ISO file is, simply put, a digital file format replicating a physical CD, DVD, or BD. When the Options icon appears, you will also see a list of volumes next to it that you can select. When you see your Mac has powered down, hold down the power button until you see a prompt that says “Loading startup options.” Here’s how you change the startup drive from recovery mode with an M1 or M2 Mac: This is a bit more complicated with an M-series Mac than an Intel one, where you could simply hold down the Option key while restarting and select a drive (unless you had turned on certain security settings, in which case you’d need to use recovery mode to disable them). You can also use recovery mode to change the startup disk. (Big Sur and later versions of macOS invisibly divide a macOS into a volume containing system files and a volume with your user data the data volume may not unmount correctly.) You might prefer to shut down at that point, unplug the external drive, start up again, and plug it in. You’ll have to unmount the external drive after the restart is complete, and some people have reported that macOS says one of its partitions remains in use.

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