1)
What is the meaning of "Mizrachah"?
1.
Rashi: [Efrayim received] what is to the east [of the line between Atros Adar and Beis Choron Elyon].
2)
In whose portion was "Beis Choron Elyon"?
1.
Bi'ur ha'Gra: It was of Menasheh. 1
2.
Malbim: Both Beis Choron Tachton and Elyon were of Efrayim (Divrei ha'Yamim 1 7:24 - Sha'arah built Beis Choron Tachton and Elyon; her father was from Efrayim).