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When I connect to my mid-2010 27" iMac in Target Display Mode, fans hum and hard disks spin (with occasional I/O). Being logged out in iMac's macOS doesn't help.

Is it possible to lower this unnecessary power drag or at least minimize the noise? Can I boot iMac as a dumb-display?

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No, you cannot go into low power mode while in Target Display mode.

While your iMac is "acting" as a "passthrough" for you display signals you are not bypassing the logic board and GPU to connect to the display. The iMac display is still connected to the internal logic board and GPU and that GPU is required to display images on the screen.

The fans are coming on because the SMC in the iMac has calculated (based on input from the temp sensors) that it has reached a certain temperature threshold and it needs to cool down. Remember, your GPU and logic board is hard at work taking DispayPort and/or Thunderbolt input and converting it to displayable graphics.

If you want a "dumb" monitor, the best option is to get a dumb monitor.

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  • Or maybe I'll put up some music to hide the noise ¯\_(ツ)_/¯ As apps can continue running on the iMac while TDM is on, could I programmatically do something to alleviate, e.g. put HDD to sleep? I understand macOS will ignore most conventional sleep-like signals, but could there be any tricks?
    – Jari Keinänen
    Commented Jun 7, 2017 at 12:29
  • There's disk IO for a reason - putting it to sleep is moot - if the iMac needs to read/write something it's going to wake. Then again, by now I have migrated all my spinning disks to SSDs - even the one in my old 2007 (white) MacBook - so I don't think about disk activity much anymore. If the noise is a big issue, I would go this route and get the benefit of faster I/O
    – Allan
    Commented Jun 7, 2017 at 12:33
  • Replacing HDD with SSD has been ≈ 2 years on my to-do list, which is why I'm looking for quick alternatives ;) Thanks for all your help!
    – Jari Keinänen
    Commented Jun 7, 2017 at 12:39
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    Go for it! The 2010 iMac uses magnets to hold the display in unlike my 2012 (glue) You are looking at a 15 minute job and fairly inexpensive. I used a Samsung 850EVO in mine.
    – Allan
    Commented Jun 7, 2017 at 12:42

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