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.