Check the nightly changelog, there might be problems with the USB storage in this build.
For youtube, try reinstalling the app.
You can also flash a CM7 based custom rom, it will give better performance, features and looks as compared to the default CM nightly.
Samsung GT-S5830 Galaxy Ace
ROM: AOKP 4.2.2
Scripts: CronMod INT2EXT4+
245MHz - 800MHz, SmartassH3