On Windows, you can create a "Two-way mirror" type storage space (which allows you to create a software RAID 1).
If you start to run out of space on your "RAID 1 storage" storage space, you can add a physical disk to your storage pool to enlarge it.

Warning : adding physical disks to your storage pool will not change the fault tolerance of this "Two-way mirror" type storage space.
Indeed, even with 3 or 4 physical disks, you will still be protected against the failure of a single physical disk.
If you really want to increase the fault tolerance, you will have to use a "Three-way mirror" type storage space (software RAID 11) which allows you to tolerate the failure of 2 physical disks at the same time.

Tutorial made on Windows 10 version 2104 and also tested with Windows 11 v21H2, 8.1 Pro and 8 Pro.

Warning : although enlarging a "Two-way mirror" type storage space (software RAID 1) is also possible on Windows 8 and 8.1, the optional option to optimize the use of drives is not available on these 2 versions of Windows.

1. Create a two-way mirror storage space (RAID 1)
2. Add a physical disk to expand your storage pool
3. Increase the size of a two-way mirror storage space (RAID 1)

1. Create a two-way mirror storage space (RAID 1)

On Windows, you can create a "Two-way mirror" type storage space.
As you can see, Windows tells you that this type of storage space writes 2 copies of your data.

Moreover, you will notice that the size indicated by default for the size of this storage space is half the capacity of the available pool.
In our case, with 2 physical disks of 500 GB, we will be able to store 498 GB of data.
Including resilience (fault tolerance), the value displayed by Windows is 996 GB (which therefore includes the copy of these 498 GB of data).

As expected, with 2 physical disks, the size of your storage space is the same as the size of the smallest physical disk used.

In file explorer, our "RAID 1 storage" is currently 497GB in size (about 500GB).

To make it clearer in the rest of this tutorial, we have stored about 25 GB of data.

Now, in the "Storage Spaces" section of the control panel, we can see that the used pool capacity is 51 GB (= about 2 x 25 GB of data).
Which is normal since with this type of storage space, Windows stores the same data on the 2 physical disks of your storage pool.
So, if a physical disk fails, you will not lose anything.

You can also see that the percentage of use of each disk has increased and is identical.

2. Add a physical disk to expand your storage pool

If you are out of space on your "Two-way Mirror" storage space, you can increase the available disk space by adding a physical disk to your storage pool.
To do this, turn off your computer, plug an additional physical disk into your computer, then turn on your computer.

Once your computer is turned back on, go back to the "System and Security -> Storage Spaces" section of the Control Panel and click on the "Change settings" button (at the top of the window) to obtain administrator rights.
Then, click on the "Add drives" link visible at the top of your storage pool.

In the "Unformatted drives" list, select your new physical disk.
Then, preferably, check the "Optimize drive usage..." box (available since Windows 10).
Then, click "Add drives".

Your new physical disk will appear at the bottom of the "Physical drives" list in your storage pool and the message "Optimizing drive usage is complete at 0%" will automatically appear.

Please wait while optimizing drive usage.
This process helps distribute the data in your storage space evenly across the different physical disks in your storage pool.
This will improve the performance of your storage space (when this process is complete).

Once the drive usage optimization is complete, the message will disappear and you will see that the usage percentage of your physical disks will be more or less the same.

However, if you go to file explorer, you will notice that its size has not changed (despite adding a physical disk to your storage pool).

3. Increase the size of a two-way mirror storage space (RAID 1)

To be able to use the new disk space on your new physical disk, you will need to change the configuration of your storage space.
To do this, next to your "Two-way Mirror" storage space, click on the "Edit" link.

By default, the size shown in the "Storage size (maximum)" box corresponds to the current size of your storage space.

To prevent Windows from telling you one day that the available space on your storage space is not enough and that you need to add a physical disk again, you need to make sure that the "Including resiliency" value does not exceed the "Available pool capacity" value.
However, Windows only allows you to change the "Storage space size (maximum)" value on which the value displayed as "Including resiliency" is based.
So you need to find the ratio that Windows uses to calculate the value that you will have to indicate as "Storage space size (maximum)".

The easiest way to do this is to temporarily indicate the value "2" (GB) in the "Storage space size (maximum)" box.
This will display the value "4.00" for the resilience inclusion.
If you divide 4 (Including resiliency) by 2 (Storage space size (maximum)), you will quickly find that the ratio is 2.

Now, since the "Including resiliency" value should never exceed the "Available pool capacity" value, you just need to divide the "Available pool capacity" value by the ratio of 2 that you just found.
Note that this ratio is specific to the "Two-way mirror" type (software RAID 1) used here.

In our case, we divide 1.41 (TB) by 2 and we find the value 0.705 (TB).

Enter the calculated value in the "Storage space size (maximum)" box.
In our case: 0.705 TB.

As if by magic, you see that the "Including resiliency" value also changes and that it is very slightly lower than the "Available pool capacity" value.

Once the value is changed, click on the "Change storage space" button.

Important : it is important to write the value found in the same unit as the one indicated for the "Available pool capacity".
Otherwise, you will lose space because 1 TB = 1024 GB (and not 1000 GB).

As expected, the size of your storage space has been changed.
Note: in our case, a capacity of 722 GB appears instead of the value 0.705 TB that we had indicated previously.

If you multiply the previously given TB value by 1024 (because 1 TB = 1024 GB), you will find approximately this value.
In our case: 0.705 TB x 1024 = 721.92 GB.

If you go to file explorer, you will see that the size of your "RAID 1 storage" has also changed.
In our case, our "RAID 1 storage" is now 721 GB instead of 498 GB.

As you might have guessed, the data that was there has been preserved.