I attempted to install the latest Catalina supplemental update on my 2019 iMac, however, after it downloaded and the Mac restarted it got stuck on the update screen, and it's been that way for 2 days.

I tried restarting and resetting the parameter RAM, which didn't really do anything, and it won't boot into Safe Mode; the system restarts right back into the update screen.

I booted up from an external drive running Catalina, hoping that I might be able to copy files to another external. I can see the Macintosh HD volume in the Finder, but not the Macintosh HD - Data volume, so I can't access my files.

I then booted up an older iMac and put the 2019 iMac into Target Disk Mode, but it's basically the same problem: Macintosh HD - Data shows mounted in Disk Utility, but I don't see it in the Finder.

System Integrity Protection is NOT enabled.

Any suggestions on how to mount Macintosh HD - Data via Target Disk Mode or from an external drive?

  • Go to Finder, then Cmd comma for prefs, then General - is External Disks checked?
    – Tetsujin
    Aug 22, 2020 at 15:58
  • @Tetsujin Yes it's checked; still can't see the Data volume. Edit: the Data drive also shows as mounted in Disk Utilities, FWIW.
    – timgavin
    Aug 22, 2020 at 18:18

2 Answers 2


For those who might still have this problem when in target disk mode. Right click on the Macintosh HD - Data volume in Disk Utility and click "show in finder"


I've seen this before, sometimes the updates hiccup and fail to bless the startup volume after an update. So it keeps booting from the update volume and it fails because it already moved files and cannot locate them.

You should be able to boot into the internal drive Recovery Mode by holding CMD+R on power up. (disconnect the external drive)

Then see if you can get to the pull down menus in Recovery Mode and select Start Up. Set your Macintosh HD as your start up volume and reboot. It might just come back up all normal and nice like it's supposed to.

Short of that fix, there could be something more complicated happening. But I've performed this fix several dozen times over the last few years and it typically works. So try this first. You did not say that Recovery Mode was not working.

If Recovery Mode on the internal disk does not boot, then when you power on the iMac while holding the T key that will put you into Target Disk mode. Attach a thunderbolt data cable to another Mac and it should be able to mount the drive on the second Mac. If you encrypted the drives you may need to unlock them first. You would only see the two distinct volumes (system & data) if you are looking at it from a lower level like the Terminal diskutil, etc. Finder will show it as only Macintosh HD. In the Finder Preferences you can enable showing External drives on the sidebar and on the Desktop as Tetsuhin mentioned.

But seriously, try booting from Recovery Mode on the internal drive of the iMac with the problem and forcing the startup disk from the drop down (utilities) menu first. That almost always works.

  • Thanks. I can boot into Recovery and select the startup disk, but when I click Restart I get the spinning beach ball, so I have to hold the power button to reboot, at which point it goes into a loop of starting up, then, I think rebooting. It's hard to tell since they removed the startup chime. As I stated in my OP, I can access the HD volume, but not the Data volume when in target disk mode, even with Show External Disks checked. However I CAN see the Data drive in the Terminal but I can't cd to it. -bash: cd: Macintosh: No such file or directory
    – timgavin
    Aug 22, 2020 at 18:16
  • If, while in Target Disk Mode, I open the Macintosh HD volume in the Finder, then go to System > Volumes > Data, it's empty. However Disk Utilities shows the Data volume has over 400GB on it, as it should.
    – timgavin
    Aug 22, 2020 at 18:27
  • Is the "Macintosh HD - Data" drive mounted in Disk Utility? Is it encrypted with FileVault? Do you have a Fusion drive in the iMac? Aug 22, 2020 at 19:06
  • Yes, it's mounted in Disk Utilities. No, it's not encrypted. Yes, it's a fusion drive.
    – timgavin
    Aug 22, 2020 at 20:02
  • Ouch... Fusion is a problem. Hope you have a backup because if either the SSD or the HDD has a fault your data is toast. Aug 22, 2020 at 20:06

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