Besides vs. Except FROM Pearson and Longman Grammar Q & A

Additionally, when apart from or except for (NOT besides) introduce exceptions to generalizations, they mean the same thing: they mean that what follows immediately is the only exception to the category. You can not use "besides" (as in (n) and (p) below) to introduce the exception first in a sentence, as you can in (m) and (o):

(m)	Except for / Apart from his asthma, the Olympic swimmer is in perfect health.
(n)	NOT POSSIBLE: Besides his asthma, the Olympic swimmer is in perfect health.
(o)	Except for / Apart from the early morning flight, all the flights to California make a stop in Chicago.
(p)	NOT POSSIBLE: Besides the early morning flight, all the flights to California make a stop in Chicago.