This afternoon my 2017 iMac 5K crashed. Upon rebooting, it hung up with the progress bar about halfway across.
System: 2017 iMac 5K, macOS 11.6.4, 1TB SSD (added in response to @MacGuyMI)
Booted into recovery mode. Disk utility shows the internal SSD as “uninitialized”.
Attempted to reinstall the OS. When the Big Sur installer got to the screen where it says “Select the disk where you want to install macOS” there’s just a big blank space.
Attempted to reformat the drive in Recovery Mode using Disk Utility. It quickly gives the error message, “Unable to write to the last block of the device. : (-69760)”
Reboots now give the blinking gray folder with a question mark.
Booting into recovery mode now seems to go to internet recovery rather than the recovery partition. Attempts to reinstall or reformat still fail.
Tried resetting NVRAM. No change.
Disconnected everything except the keyboard and Ethernet. No change.
Removed the two sticks of aftermarket OWC RAM. No change.
Attempts to boot from a clone backup hang up with the progress bar at about 50%
My conclusion is that at the very least the SSD is toast. The inability to boot from the clone backup could indicate additional problems beyond the SSD. That said, I’d welcome any insight from other Mac Power Users who’ve encountered anything similar.
I guess my interest in what new Macs Apple might announce tomorrow has suddenly gotten a lot more urgent. Maybe the iMac gave up the ghost because I talked about buying a new desktop.
I think @ACautionaryTale is proposing something slightly different: rather than booting from my existing clone, use recovery mode to create a fresh macOS install on an external disk and try to boot from that.
Searching on Google for ( error “-69670” ) turned up quite a few suggestions, most involving external drives. However it appears that forcing the drive to unmount may then allow you to reformat the drive.
I’ve never had this problem, but I’ve used diskutil to solve other problems from time to time. Since you can get into recovery mode this might be something to try. Good luck.
For now, my temporary solution is to use my 14" MacBook Pro with a couple of external monitors (glad I bought that OWC Thunderbolt 3 Dock). Problem is, the camera I use for video calls is mounted on my iMac. Since my long-term plans are so unsettled I decided to just slide the carcass of my dead iMac back a little bit and put one of my external monitors in front of it.