I have a 2009 iMac which was in my son's possession. Somehow the OS was wiped clean. I'm trying to reinstall High Sierra and have created a bootable disk on a flash drive. When it comes time to select the drive for installation, the only choice given is the flash drive disk...which says "this disk it is locked."
iMac 27″
Go a few steps back. Boot from the usb drive, but select disk utility first. After that follow this instructions to see if you can repair your disk
How to repair a Mac disk with Disk Utility - Apple Support
I had this also with a 2009 iMac, what helped me was to plug in an external hard drive and install high Sierra on that one. When it was installed, I opened disk utility and I could re mount my internal drive. So if you don't see your internal drive in above steps or you cannot re-mount it, you can try that as last resort
Go a few steps back. Boot from the usb drive, but select disk utility first. After that follow this instructions to see if you can repair your disk
How to repair a Mac disk with Disk Utility - Apple Support
I had this also with a 2009 iMac, what helped me was to plug in an external hard drive and install high Sierra on that one. When it was installed, I opened disk utility and I could re mount my internal drive. So if you don't see your internal drive in above steps or you cannot re-mount it, you can try that as last resort
Reinstall High Sierra on 2009 iMac
