a. She might come tomorrow.
b. She might be able to come tomorrow.
In sentence (b), does it mean that there are some external reasons preventing her from coming, but she probably managed to solve the external problems and then come tomorrow,
whereas in sentence (a) there is no external reasons preventing her from coming, only her internal will prevents, if any.
am I right?
Yes for b). For a), your explanation works, but there could be other factors, including external ones- we don't necessarily know what could affect whether she comes or not.