macOS
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To preserve space for another volume, you can either set its Reserve size, or limit the backup volume with a Quota size.
> Store files on your backup drive
What’s the best way to store other files on my Time Machine backup drive?
There are several options, depending on whether you want to limit the space taken by the backups or your additional volume. Unfortunately, you have to decide their size and configuration when you first set them up, because they can’t be changed at a later date without putting their contents at risk.
The strictest and most inflexible option is to create a second container using Disk Utility and create your extra volume in that. When partitioning the drive, you set the size of each container, which you don’t intend to change. You can still change partition sizes later, but should avoid that, as it may not be possible without erasing one of those containers.