I have an Airport Extreme with an attached USB drive that I've been accessing via Back to My Mac using these instructions (see the section with the text, "...access files on a hard drive connected to an AirPort Extreme...").
Recently I extended (over Ethernet) my Airport network using an Airport Express upstairs. i.e.
Airport Extreme with USB drive (downstairs) <--Ethernet--> Airport Express (upstairs)
I used these instructions (see the first post by Bob Timmons) to extend my network. I can tell that the network extension is over Ethernet because AirPort Utility shows both Airports connected to each other with a solid line (extension over WiFi uses a dashed line).
However, this makes my downstairs Airport Extreme disappear from Back to My Mac. Using Bonjour Browser on a remote Mac, I can see only the upstairs Airport Express:
#######.members.btmm.icloud.com - 4 Airport Base Station (_airport._tcp.) - 1 airport_upstairs Apple File Sharing (_afpovertcp.tcp.) - 2 home_mac_1 home_mac_2
Notice that only airport_upstairs is visible via Back to My Mac (even though airport_downstairs is the one configured for Back to My Mac). Has anyone else run into this problem--do you know how to fix it?