Searching the many helpful threads on this topic didn't answer my particular question, on something that should be pretty simple. What I'm trying to do: Set up a home network system so that a MacBook Pro (late 2012) can do things like read/write to the main computer in the house (iMac, 2013), use the external hard drive attached to the iMac for Time Machine backup, print, etc. Probably more context than needed, but you never know: Our ISP (great company, quality products) provides a combined modem/router/wifi transmitter (which I'll call the modem here), which is at one corner of our (small) house. Because the house has lath and plaster walls, which are great radio-wave barriers, we have an extender, ethernet cabled to the modem, at the opposite corner, which also puts out wifi signal. The iMac is connected by ethernet cable to the extender; out the back of the iMac is a cable connecting to what I will call a splitter (because I don't know its proper name) from which cables run to the external hard drive and other hardware. Both the iMac and the MacBook Pro use the current version of Yosemite. The iMac happily uses the external hard drive for backup, prints to the printer, connects to the interweb - all that stuff. The MacBook, using either of the two wifi connections, happily connects to the interweb. What I've done so far: I've read all kinds of helpful hints on the net about how to share a drive, printer, etc. on a home network. I've then checked that the external hard drive is formatted as Mac OS Extended (Journaled) - yes. I've gone into System Preferences on the iMac for sharing and turned on sharing for the external hard drive, the main computer's hard drive, and the printer. I've gone into System Preferences on the MacBook for sharing and turned on file sharing and printer sharing. I've called the ISP's helpful staff and confirmed that the network seems to be working; I've reset the modem by unplugging it and watching all the lights perk on again when I plug it in again after a minute or two. Results: I can't see the iMac, external hard drive, or printer from the MacBook; I can't see the MacBook from the iMac. I can see the dark clouds out the window on this rainy day and recognize what a great metaphor they are for how I'm feeling right now. What steps have I left out?