Hey there, let's break down your questions clearly—this is a common confusion when setting up VMs, so you're not alone!

How does the 2TB slider affect your physical host disk space? #

First off, let's clarify the key point: the slider shows the maximum virtual size your VM's disk can reach, not the space it immediately takes from your host. Here's what that means for your 60GB free space:

• By default, VirtualBox creates dynamic (thin-provisioned) disks. This means the actual file on your host starts tiny (just a few megabytes) and only grows as you add data to the VM. Even if you slide it to 2TB, your host will only use space proportional to what's stored in the VM—up to the limit of your 60GB free space (since you can't write more data than your host has available).
• If you had chosen a fixed (thick-provisioned) disk, VirtualBox would try to allocate the full size you select upfront. But in your case, trying to set a fixed disk larger than 60GB would throw an error, because your host doesn't have enough free space to reserve that much.
• So the 2TB range is just the maximum virtual disk size VirtualBox supports—it doesn't force your host to use that space unless the VM actually needs it (and your host has it available).

The "science" behind virtualizing physical storage for VMs #

VirtualBox uses disk image files (like .vdi for its native format) to act as virtual hard drives for your VM. Here's how the storage is managed:

Dynamic (Thin-Provisioned) Disks #

• Initial Creation: When you make a dynamic disk, VirtualBox creates a small placeholder file on your host. This file only contains metadata about the virtual disk's structure, not actual data.
• On-Demand Allocation: As your VM writes data (saves files, installs apps), VirtualBox allocates chunks of space (usually 1MB or 4MB blocks) from your host's free storage to the virtual disk file. It keeps a mapping table that links each block in the virtual disk to a physical sector on your host's drive.
• Efficiency: This is great for saving space because you only use what your VM actually needs. The downside is a tiny bit of overhead when new blocks are allocated, but it's barely noticeable for most use cases.

Fixed (Thick-Provisioned) Disks #

• Immediate Allocation: When you create a fixed disk, VirtualBox immediately reserves the full specified size on your host. It fills the unused parts of the virtual disk with zeroes (or uses sparse allocation on some file systems, but the space is still reserved).
• Performance: Fixed disks have slightly better performance because there's no overhead of dynamically allocating blocks during writes. This is useful if you need consistent disk speed for things like database servers or high-I/O workloads.

Key Mapping Layer #

The hypervisor (VirtualBox, in this case) acts as a middleman between your VM and the host's physical storage. Every time the VM sends a read/write request to its virtual disk, VirtualBox translates that request to the corresponding physical sectors on your host's drive. This abstraction lets the VM treat the disk image as a real physical drive, while the host manages the actual storage efficiently.

Important Note #

You can't "overcommit" space forever. If your VM tries to write more data than your host has free space left, it will hit a "disk full" error—just like a physical computer running out of space. So even if you set a 2TB virtual disk, you're limited by your host's actual available storage.

备注：内容来源于stack exchange，提问作者Nabin