Question :
Since the latest version of OSX (entitled “macOS Sierra) has been yesterday, I wanted to install it on my Mac. I clicked on “download” in the Mac AppStore and  the download started ; on the dedicated product page I could see the status bar filling up and finally stating “downloaded”… but then, nothing happened…
The download wouldn’t install…
I re-checked if macOS Sierra does support my Mac, it does. So I tried restarting the AppStore-app, I tried logging out my iCloud/AppStore-account & than logging back in, I tried rebooting my Mac, I tried rebooting in SafeMode, I tried FirstAid in the Disk Utility app, I tried clicking 10 times on the “downloaded” button repeatedly, I tried installing the DeBug-menus & cleaning the AppStore cache, I tried deleting the hidden AppStore downloads folder… and I tried various command line solutions found online in the Terminal app… nothing helped… I am still stuck with the button under the macOS Sierra icon saying “downloaded”… and nothing happens…
What can I do ?
Answer :
You are probably not (really) going to like this answer, because this will set you back hours & hours, in which you can’t use your Mac ; but it’s the only way known to solve this, and you need to have a recent TimeMachine backup (or alike) for it :
- shut down your Mac
- disconnect all cables  from your Mac (except your keyboard & mouse if you haven’t got a MacBook)
- restart your Mac, and be quick to press the [CMD]+[R] key-combo during startup (your Mac will now start up in Recovery Mode)
- in the “OS X Utilities” window that appears, select Restore From TimeMachine Backup and click Continue
- in the next “Restore Your System” window, click Continue
- now first make sure that your Mac can connect to your TimeMachine-harddisk by plugging in it’s USB-cable, plugging in your Ethernet-network-cable or connect to your WiFi router
- then select the hard drive that has your TimeMachine backups on it and click Continue
- in the next window, make sure you select the most recent backup that is from before you tried to download macOS Sierra ; only if you’re sure you’ve selected the right backup, proceed and click Continue
- now wait (or do something more useful in the mean time) and wait and wait ; this is going to take hours and hours ; putting back a TimeMachine backup this way will go a a speed of about 30BG per hour, so if your hard drive is 500GB it may take you about 15 hours (yes, an entire night and an entire morning…)
- finally, once the TimeMachine backup-reinstall is done, your Mac will restart… just let it
- then, when logged in, open the Mac AppStore-app and notice that the button under the macOS Sierra now says “download” ; click on it
- after the download has completed, you should now see the installer window popup on your screen… proceed from there…
that’s it… it’s simple, but very time-consuming…
enjoy !

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