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I was reinstalling the OS on my Mac via Internet Recovery using a wifi connection. As it wasn't connected to a power source, the screen of my Mac would constantly go black and I would press the screen brightness button in order to make the screen light up again so I could check the progress. I did this a lot, and nothing seemed wrong with the numerous times I did this between the start of the download and the 46 minutes remaining mark - it was incremental and the filling of the progress bar seemed to fit with how long was left.

However: when the progress bar was 2/3s full and there was 45 minutes remaining on the download, the screen went black and a split second after it did when I pressed a button to light the screen up again, the download had apparently gone from 45 mins to 0 mins remaining. That stage where it's about to restart and go to the black screen with the Apple logo "installing: ## minutes remaining..." page.

I know the minutes remaining download estimate can be wildly inaccurate sometimes (a previous time waiting for Internet Recovery functions to download would've apparently taken 24 hours hah), but has this happened to anyone else before? Where the progress bar stops filling and the minutes remaining estimate doesn't go down even if the download is apparently still going? Have I somehow downloaded a compromised version of the OS?

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    Most likely the Mac was blanking the screen to conserve battery power. As for the compromised version of OS X, this is highly unlikely. Do you have any evidence that your wifi network is compromised? – IconDaemon Nov 29 '15 at 15:00
  • I've noticed many times after I wake my iMac from sleep that the Menu bar clock will advance to the correct time after the screen lights up. Perhaps Mac OS X, or the graphics card itself, keeps a cached version of the screen in a buffer somewhere when sleep occurs, and the screen is then refreshed when the rest of the HW wakes up. I would say there is no cause for alarm. – IconDaemon Nov 30 '15 at 1:48

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