display – How do I access my iMac in Target disk mode from Terminal running on my Firewire-connected MBP, to troubleshoot iMac possible rEFInd boot problem?

I want to uninstall rEFInd in a peculiar situation.

I overheated my ageing iMac doing a Zoom chat while running other programs. It crashed. Since when it won’t reboot and there are lines on the screen. The GPU may be fried. BUT following various searches I successfully fsck -fy ‘d itand it rebooted as normal. While it was working I verified the install with DiskWarrior (kudos). It is fine. Then it fritz’d again and won’t now reboot. I get to a blank white screen with lines on it. I can see Apple log and progress bars.

Before splashing on a new GPU or new iMac I want to remove rEFInd to eliminate the possibility of graphics driver corruption or other boot issues on the iMac.

Below is what I have tried so far.

I CAN

•   boot iMac into Target mode
•   boot iMac into single-user mode - in this case the screen has lines on it but is basically usable. 

I CAN’T

•   boot into Recovery mode (either by booting with R or Alt - Alt shows the Recovery Disk and booting it shows a progress bar, and - R shows a progress bar but on both cases it goes nowhere)
•   boot into Hardware test mode

In Target mode connected to my MBP. I can see all the iMac files in the MBP’s Finder. But I can’t see anything below the Mac OS.

In single user mode I see the following before the final root prompt:

Root device is mounted read only
Enabling and disabling services is not supported in single-user mode, and disabled services will not be respected when loading services while in single-user mode. 
To mount the root device as read-write:
>$/sbin/fsck -fy
>$/sbin/mount -uw / 

Doing

$/sbin/mount -uw 

(only) results in

root_device on / (hfs, local, read-only, journaled)
devfs on /dev (devfs, local, no browse)

and the commands below have the same results as shown below, so AFAICS either it didn’t mount as -r-w or I made some other error.

In single-user mode

localhost:/ root# ls 

produces s list which includes Volumes but does not include EFI.

localhost:/ root# cd Volumes 
localhost:Volumes root# ls

shows

Linux   Macintosh SSD

but not EFI. However, EFI is present on its own partition. Diskutil will not run in single-user mode, but it will run in Target mode, and it shows that the EFI partition is present on the iMac SSD.

In Target disk mode via Terminal on the MBP with the iMac connected by Firewire I get:

Ric0sMBP% diskutil list
/dev/disk0 (internal, physical):
  #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS MacBookSSD500gb         371.5 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
   4:           Linux Filesystem                         127.6 GB   disk0s4
/dev/disk1 (internal, physical):
  #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *127.9 GB   disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:       Microsoft Basic Data HD                      127.5 GB   disk1s2
/dev/disk2 (external, physical):
  #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS Macintosh SSD           799.5 GB   disk2s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk2s3
   4:       Microsoft Basic Data Linux                   199.7 GB   disk2s4

The iMac is /dev/disk2.

Ric0sMBP% ls /Volumes
HD      Linux       MacBookSSD500gb Macintosh SSD   iMACtemp
Ric0sMBP% 

The iMac is “Macintosh SSD”. iMACtemp is a temp directory I made for the next step.

Ric0sMBP% sudo mount -t hfs /dev/disk2 /Volumes/iMACtemp
Password:
mount_hfs: error on mount(): error = -1.
mount_hfs: Resource busy
Ric0sMBP% cd /Volumes/Macintosh SSD
cd: string not in pwd: /Volumes/Macintosh
Ric0sMBP% 

So …

I can’t access the iMac EFI partition via Terminal on the MBP, even though I can see it is there. And I can’t mount the disk to -rm it

And I can’t use the MBP Finder to delete it, because Finder doesn’t see it.

And I can’t access it from the iMac desktop because that won’t boot.

And I can’t find the EFI partition on the iMac in single-user mode.

Finally (!)

I have a TimeMachine backup of the iMac’s OSX partition on an external Firewire drive. That does not contain the EFI partition (either on the external drive or on any folders in the backup). So I guess I could wipe the entire SSD on the iMac, reinstall the OS from a recovery USB (which I have) and reinstall the TM backup onto that.

But in order to wipe the drive, I would need to be able to mount it in Terminal. I think.

If I try to reinstall the OS from USB without deleting the EFI partition, I think I will still have the same problem. At least, I can’t know that if the problem persists, it is not down to something wrong with the EFI.

BTW while the iMac was still working I did a reinstall of the OS (alone), but that didn’t prevent the second crash and didn’t enable Recovery mode.

That’s the limit of my know-how.


Can anybody tell me how to remove rEFInd from the iMac SSD, see if that fixes the boot / graphics issue, and proceed from there?

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