The risks of partitioning a Mac
The only real risk when you partition your hard drive is data loss. You can mitigate this risk by backing up or cloning your disk before you start and whenever you re-size partitions.
How to extend macOS partition: Resize and change your partition size
Through Disk Utilities you can also change the size of your partition, where you can extend it and sometimes even shrink it, depending on the amount of data free on your drive.
To change the size of the partition, simply click on the partition segment within Disk Utilities and press the plus '+' or minus '-' button to change its size.
If you're removing the partition and want to re-allocate the space to your primary drive, then you'll need to first click on the partition and select Erase, so make sure you have a backup of the data. Once you've deleted all the data on the drive, click on the partition again and press the minus '-' button. This will now re-allocate the disk space to your primary drive.
If you're extending the partition, make sure you've got enough space on your primary drive to accommodate the changes in size. Simply press the plus '+' button to extend the size of the partition.
Once you make changes, you Mac will take a little while to apply the changes. As soon as the process is complete you'll be presented with a green tick indicating the 'Operation sucessful'.
The alternatives to partitioning a Mac
If you don't want to partition your main disk, there are other ways you can safely run a different version of Mac OS X or a beta of a new version. The simplest is to install it on an external hard drive, or even a USB stick. You can then boot from that, either by selecting it as the Startup Disk in System Preferences, or by holding down the Alt key during startup and selecting it when prompted.
If you want to run Windows without partitioning your hard drive, you can use Parallels Desktop or VM Ware Fusion to create a virtual environment.
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