Hello, Franco_896
Welcome to the Microsoft Community.
Hello, thank you for your feedback on your issue here, it seems that you want to resize the partition on the 960 GB SSD, you can use the built-in tool.
Below is a general guide on how to modify partitions:
Open Disk Management:
Press Win + X and select Disk Management, or type diskmgmt.msc in the Run dialog box (Win + R)
Locate the SSD:
In the Disk Management window, find the SSD in the list of drives.
Resize the partition:
Right-click on the partition you want to resize (e.g., 300 GB partition) and select Extend Volume or Shrink Volume, depending on whether you want to increase or decrease the partition size
Follow the prompts to specify the amount of space to be added or removed.
Note: The Extend Volume option is only available if there is unallocated space next to the partition to be extended. Similarly, Shrink Volume creates unallocated space if the partition size is reduced.
Also, if you encounter failure or error report during the operation please share here and I will check it for you.
Best regards
Bobhe | Microsoft Community Support Specialist