Resizing the Disk of a UTM Virtual Machine with macOS

A Detailed Guide to Expanding Your Virtual Disk Space

Key Insights

Backup and Preparation: Always ensure you have a backup of your virtual machine before making disk alterations.
Two-Phase Process: The resizing process involves adjusting the virtual disk within UTM and then expanding the file system within macOS using Disk Utility.
Technical Adjustments: Resize the virtual disk file (commonly a .qcow2 file) and manually expand the partition to allocate the extra space.

Overview

Resizing the disk of a UTM virtual machine running macOS as the guest operating system is a multi-step process that involves modifying both the virtual disk image and the partition scheme within macOS. The overall strategy is divided into two main parts: first, adjusting the disk size in UTM’s settings; and second, either expanding or creating partitions inside macOS using Disk Utility. This guide provides an in-depth explanation backed by community knowledge and official documentation references, ensuring that the instructions are reliable and safe to follow.

Detailed Steps

Phase 1: Resizing the Virtual Disk in UTM

Step 1: Power Down the Virtual Machine

The very first step in any disk modification process is to completely shut down the virtual machine. This prevents data corruption and ensures that the resizing operation can be performed safely.

Action: Ensure that the macOS guest system is properly shut down rather than simply paused.

Step 2: Open UTM and Access VM Settings

Launch UTM and locate the virtual machine for which you wish to increase the disk size. Accessing the settings is typically done by right-clicking the particular VM in the sidebar and selecting "Edit".

Action: Right-click on the target VM and choose the "Edit" option to open its configuration settings.

Step 3: Locate the Drive and Initiate Resizing

Within the VM settings, navigate to the "Drives" section. The disk you need to resize is typically identified as a VirtIO drive and is associated with a .qcow2 image file. Click on the drive setting to reveal additional options.

Action: Click the “Resize” button associated with the disk in question. You will then be prompted to input the new desired size.

Step 4: Enter the New Disk Size and Confirm

Safely input the new disk size. This new size should be larger than the current allocation, as this expansion will eventually allow more data storage in the macOS guest system.

Enter the new capacity (for example, if your current disk is 50GB, consider increasing it to 100GB based on your storage needs).
Confirm the changes by clicking the dedicated “Resize” or similar confirmation button.

Action: Confirm the updated disk size; any changes made here update the virtual disk file, although they may not immediately reflect in the guest operating system.

Phase 2: Expanding the File System in macOS

Step 1: Boot the Virtual Machine

Once the disk image has been resized through the UTM interface, it is time to let macOS detect the newly available space. Start the virtual machine so that macOS boots up normally and recognizes the increased disk capacity.

Action: Boot the macOS guest system.

Step 2: Open Disk Utility in macOS

macOS includes a built-in tool called Disk Utility that allows you to manage disk partitions and file systems. Locate and open Disk Utility from within the macOS Applications/Utilities folder.

Action: Open Disk Utility to prepare for partition expansion.

Step 3: Analyze the Disk Layout

Within Disk Utility, select your primary disk (often labeled as "Apple SSD" or similar depending on your installation). The disk interface displays existing partitions and any unallocated space that resulted from the disk expansion in UTM.

Action: Review the disk partitions to determine if the unallocated space is adjacent to the primary partition. This will influence how you can expand the partition.

Step 4: Expand the Partition to Include the New Space

To make use of the new disk capacity, you must expand your existing partition. Within Disk Utility, select the partition and choose the “Partition” or “Resize” option. Drag the partition boundary to incorporate the additional unallocated space, or enter an updated size manually.

Action: Click the "Apply" or "Resize" button to confirm the changes. macOS may require a reboot or additional confirmation prompts.

Step 5: Verify File System Expansion

After resizing, macOS should now display a partition that occupies the full size of the physical disk as resized in UTM. Use Disk Utility to confirm that the file system has been expanded and that no errors were encountered.

Action: Confirm the proper utilization of the disk space. If necessary, use the “First Aid” function in Disk Utility to ensure the integrity of the file system.

Additional Considerations & Troubleshooting

Backup and Data Integrity

Before beginning the disk resizing process, it is crucial to back up your virtual machine to prevent potential data loss. Disk operations, especially involving partition modifications, carry inherent risks. A backup ensures that you can restore your system if something does not go as planned.

Tip: Use UTM’s backup features or manually copy the virtual disk file to a secure location.

Handling Sparse Files and Partition Layouts

When dealing with .qcow2 images, the disk file format often uses sparse file allocation, which allows the disk size to show actual allocated data rather than the maximum capacity. The resizing process expands the maximum file size, but you must ensure that the partition frontend is properly expanded to fill the new allocation.

Note: For highly intricate partition setups, consider third-party partitioning tools, though macOS’s Disk Utility is sufficient for most users.

File System Limits and Compatibility

macOS typically uses the APFS (Apple File System) for its storage volumes. APFS handles dynamic storage allocation well, but it is important to ensure that the changes made via Disk Utility propagate without error. Carefully monitor the Disk Utility logs and confirm through system information that the space is reported accurately.

Reminder: Ensure operating system updates are not pending, as these could potentially interfere with disk management operations.

Common Issues and Their Solutions

Some users may experience issues after resizing either due to corrupted partition tables or improper handling of the resized disk image. Should you encounter such problems, consider the following approaches:

Revisit the partition tool in Disk Utility to attempt a manual resize again.
Run Disk Utility's “First Aid” function to repair any minor disk errors.
If issues persist, consider restoring the backed-up disk image and retrying the procedure.

Summary Table of Steps

Step	Description
1	Power down the virtual machine
2	Access UTM VM settings and locate the disk drive
3	Click on "Resize" and enter the new disk capacity
4	Boot the macOS guest operating system
5	Launch Disk Utility in macOS
6	Expand the partition to occupy the unallocated space
7	Verify and repair the file system if needed

Technical Precautions & Best Practices

Backup Your Data

Before starting any disk resizing activity, create a full backup of your virtual machine. This precautionary measure safeguards against the risks associated with modifying disk configurations and ensures that you can revert to the previous state in case of errors.

Documentation and Community Forums

The process outlined here is supported by multiple sources, including official UTM documentation and community forums such as Reddit and GitHub. Reviewing these can provide additional context and help troubleshoot any unexpected issues.

Sequential Process Execution

It is essential to perform each step sequentially and only proceed once the current operation is confirmed successful. Avoid jumping ahead in the process as doing so could lead to incomplete updates or system inconsistencies.

Follow-Up Actions After Resizing

After successfully resizing and updating your disk partitions, monitor your macOS system for any disk-related discrepancies. Performing routine checks using Disk Utility will help you catch potential issues early and ensure that your new disk allocation remains stable.