Write a Python function `find_min_diff(arr)` that takes a list of integers `arr` and returns the minimum difference between any two elements in the list. The function should handle cases where the list contains at least two elements.
#### Example Usage
```python [main.nopy]
print(find_min_diff([1, 5, 3, 19, 18, 25])) # Output: 1
print(find_min_diff([4, 3, 2, 6])) # Output: 1
print(find_min_diff([30, 5, 20, 9])) # Output: 4
```
#### Constraints
- The input list will contain at least two integers.
- The integers can be positive or negative.
- The function should return the minimum absolute difference between any two elements.