Here is some great information on how to delete the $WINDOWS.~BT folder.
It looks like when your upgrading to Windows 10 this hidden folder is created and it's where the root files of Windows 10 are stored. After the upgrade, the folder still remains there just taking space.
This folder is just there just in case you decide to rollback to an old version of Windows. But in the long run, the folder just takes up a lot of space.
If you want the best performance for your Windows OS, the more space you have the better.
Here is more information on how to remove the OS upgrade folder.
Link
It looks like when your upgrading to Windows 10 this hidden folder is created and it's where the root files of Windows 10 are stored. After the upgrade, the folder still remains there just taking space.
This folder is just there just in case you decide to rollback to an old version of Windows. But in the long run, the folder just takes up a lot of space.
If you want the best performance for your Windows OS, the more space you have the better.
Here is more information on how to remove the OS upgrade folder.
Link
