In a recent post to his Tech Field Day blog, IT consultant Stephen Foskett runs down a good number of tips and lessons about repurposing his 4-year-old iMac into a monitor for his MacBook Pro Retina Display and into a simple file server.
I have found that a number of iMac users — especially former Windows users — may not be aware of the iMac's easy A/V expansion via support for DisplayPort or that one Mac can boot another Mac as if the second box was an external hard drive. This latter capability comes from Apple's continuing support for Target Disk Mode, which is a longstanding technology on the Mac and spans SCSI, FireWire, USB and now Thunderbolt connections.
Foskett provides a bunch of good of tips for using the iMac as a laptop display and the everyday use of this useful-but-a-bit-kludgy non-KVM setup. For example, he offers suggestions on where to keep the extra, but still necessary iMac keyboard and mouse, and warns that users must take care in plugging and unplugging devices.
Note that, if you simply unplug the Mini DisplayPort cable from a closed-lid MacBook, it will immediately go to sleep. This will disrupt anything you’re doing and might be a Very Bad Thing. To keep this from happening, open the lid first, then unplug the video cable.
On the server front, Foskett said that it's easy to set up OS X as a bare-bones server without requiring OS X Server itself. He also offers that "a genuine iTunes instance is much more reliable than any alternative media server, and the same goes for a real Mac OS X AFP service."
One handy tip for using an iMac as a server is to reduce the screen resolution in the control panel. A lower-res screen is a huge help, since I’ll mostly be using it via VNC (with Screen Sharing from the MacBook Pro or Wyse PocketCloud Remote Desktop Pro on the iPad and iPhone). I dropped it to 1600×900, but could go further. It still looks fine when it’s running the iMac’s panel but is way better for use in VNC! Plus, it’s very "Inception" to show a screen sharing session to the iMac’s guts on its own panel via the MacBook Pro!