With intrinsic type T, given:

EEPROMStorage<T> first_thing(0, 0);
EEPROMStorage<T> second_thing((sizeof(T) + 1), 0);

Code such as the following:

...
second_thing = first_thing;
...

does not result in the value in first_thing being copied into second_thing, which misled me (an experienced coder) and probably would be very misleading to a novice (common among Arduino users). It in fact appears to cause the checksum of second_thing to become invalid, as evidenced by the constructor reinitializing the default value on the next boot.

Using a temporary variable yields the expected result, e.g.,

...
T temp = first_thing;
second_thing = temp;
...

I suggest that EEPROMStorage<T>::operator=(const EEPROMStorage & rhs) and EEPROMStorage<T>::operator=(const T& rhs) should be equivalent.