Will installing the latest N developer preview erase storage? I am coming from PureNexus custom ROM. I want to know if all my photos, music, and other files on my internal storage will be erased. Thanks!
Installing the whole ROM from scratch? Installing any ROM wipes the phone.
Installing the update from M to M? No.
Since yours is custom, you'll probably have to reflash a stock ROM - that will wipe everything. (It's a good idea to do a full backup - see Backing up an Android Device) before making any large change like a ROM anyway, even if it's just a major version update. It's too late after something goes wrong and you lose everything.