Minimum free doesn't keep a disk from filling up. It just keeps unRAID from writing to a disk unless it has the minimum free. If you set it to 1GB, and it has 1.5GB free, then it will write to the disk regardless of how large the file will be because it doesn't know how large the file will be.
If you have minimum set to 1GB, and you have 1.5GB free, and you try to write a 2GB file, it will fail due to not enough space.
If you have minimum set to 1GB, and you have 1.5GB free, and you try to write a 1.4GB file, it will succeed because there is enough space, but then it won't write any other files because it no longer has 1GB free.
So, you must set minimum free to be larger than the largest file you ever expect to write.