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About one month ago I upgraded my old iMac to High Sierra 13.6 without much thinking of risks involved. For a few days my iMac seemed working fine, but then it became very slow, eventually can't boot at all, showed the prohibited sign.

I tried safe/recovery mode to boot, always showed the prohibited sign! Then I decided to make a USB Mac OS installer to reinstall Mac OS Sierra because I suspected all the troubles might be caused by High Sierra's new APFS.

With the USB flash card of Mac OS Sierra installer I can boot the iMac, but from the Disk Utility I can't see Mac's internal 1TB HDD at all, so it means that there are no disk volume to install Mac OS sierra at all.

I tried to use the Mac terminal command diskutil list, it only showed my 8G USB disk, not the internal 1TB HDD.

However, when I boot the iMac with Option Key, it showed three disks: Macintosh HD, Recovery, and the USB installer. Somehow I can't boot from the first two, all would led to the prohibited sign again.

Wonder if I did something wrong above? Any ways to check the status of my 1TB HDD? Thanks for your kind help and sharing.

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    Couple of observations, but idk any fix. An 09 iMac can't run Sierra unless you force it on with DosDude's hack. I also don't think Sierra would enforce APFS on an HD, only an SSD at that time. – Tetsujin Mar 12 at 15:07
  • Does 27 mean 27 inch? I ask because Apple did not make a 27 inch iMac in mid 2009. Apple did make a 27 inch iMac in late 2009 and in this case the maximum macOS is 10.13.x. If only mid 2009, then the iMac is 20 inch and the maximum macOS is 10.11.x. – David Anderson Mar 12 at 15:51
  • Does Command (⌘)-R boot to internet recovery? If so, does the Disk Utility show the internal HDD. You may want to select Show All Devices in the View pulldown menu. – David Anderson Mar 12 at 16:14
  • As far as I'm aware, no 09 can use Internet Recovery - apple.stackexchange.com/questions/383985/… – Tetsujin Mar 12 at 17:33
  • To be candid...it doesn't make sense to go past High Sierra. A Late 2009 27" iMac had a SATA II (3Gb/s) interface which would make it a major bottle neck in terms of performance - while an SSD would extend the life, make it run cooler and quieter, you'd never get the performance you need for Mojave or later. As for APFS, if you're not running an SSD, you shouldn't be running APFS. – Allan Mar 13 at 4:27

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