How to install new OS if I don't know my user password?

Tuffy Nicolas wrote:

Thanks for the detailed reply. When I try to "Erase" the physical drive in Recovery Mode (exactly as you describe), I get an error saying "Can't Unmount Disk." Any way around this error?

Then you need to use Disk Utility to unmount that volume by selecting the volume in the left pane and clicking the "Unmount" icon on the right side.

If I choose "Reinstall OS" from the Recovery startup page, then choose the correct startup volume, and click all the install steps correctly, I get an error message from the Apple site saying "This item is temporarily unavailable. Try again later." I get this message repeatedly -- never changes. Any way around this error either?

It is probably because the certificate for the installer expired a while back. You can try changing the date to a time in 2017 (may need to be an earlier year) by using the Terminal app located on the Utilities menu. Within the Terminal app type the following to change the date & time to Jan. 2, 2017 @ 11:33am (adjust the year accordingly if you get the same error):

date  -u  0102113317

I suggest you boot into Recovery Mode using Command + Option + R as that should prevent needing to supply your AppleID & password to prove you previously purchased macOS 10.11 (Apple's information is a bit vague on this).

Keep in mind, once the drive is erased, then you will no longer be able to access Recovery Mode since the drive will be empty. You can try erasing just the "Macinotosh HD" volume in Disk Utility instead of the whole physical drive so it preserves your ability to boot back into Recovery Mode (should work if you did not have Filevault enabled).

Tuffy Nicolas wrote:

One of the other computers is a 12" Retina Macbook from 2015, which the list says IS compartible with creating an El Capitan bootable macOS 10.11 USB installer (albeit with "No Handoff Support (Bluetooth 4.0)").

Perfect.

So, would the plan then be: Create a bootable 10.11 on a USB drive, and then restart the iMac with that USB drive plugged in? ..... How could or would I be able to select the USB drive as the "startup disk"?

Create the bootable USB installer by following the instructions in the Apple article I linked previously. Read them carefully since after downloading the installer .dmg file you will need to extract the real installer to the Applications folder, after which you will run the Terminal command to make the USB installer. The article also has a link to another Apple article with links to download various macOS installers.

With the USB installer connected to the iMac, you will hold down the Option key immediately after hearing the startup chime which should display the Apple boot picker menu showing an orange icon for the USB installer (orange icon denotes an external drive). In theory if the internal drive is erased, then the Mac will most likely boot automatically to the USB stick (may take several minutes since the Mac will take time to search for bootable volumes).

Tuffy Nicolas wrote:

Further update:

I started completely from scratch, and did everything correctly, but still encountered the same mystifying problem at the very last step. To be specific:

I chose the "Install OS X El Capitan" drive, clicked the arrow; got an Apple logo, then several minutes of progress bars as the drive loaded (as expected)

Once the "OS X Utilities" window appeared, I went to the Wifi icon at the top and successfully connected to the local wifi network

In the OS X Utilities window, I choose "Install OS X"

Then the "OS X El Capitan: To set up the installation of OS X 10.11, click Continue" window appears as expected, and I clicked "Continue"

Technically not starting from scratch if you did not erase the "Macintosh HD" volume (or better yet the entire physical drive).

There is a small chance that maybe the date needs to be changed. What date is your iMac using at the moment? For local recovery mode we needed the date to be 2017 or so, but with the recent installer you downloaded, the date should either be the current date or perhaps some time in 2020. I'm not entirely sure, and I'm not sure whether the date is the problem at all, but Apple tends to have very cryptic error messages for these installers--many times an incorrect date is the problem (which date it should be is the puzzle, but for your current installer I would think it should be current -- for Nov. 9, 2022 @ 11:33am):

date  -u  1109113322

If erasing the whole physical drive causes the error, then my guess is the iMac's hard drive is failing.

You can check the health of the hard drive by creating & using a bootable Knoppix Linux USB stick. Download the Knoppix Linux DVD .iso file with "EN" in the name for ENglish. Use the Knoppix .iso file as a source for Etcher which will create the bootable Knoppix USB stick. Option Boot the Knoppix USB stick and select the orange icon labeled "EFI". While Knoppix is booting, the Mac may appear frozen on the Apple boot picker menu, so make sure to give Knoppix lots of time to finish booting.

Once Knoppix reaches the desktop, click the "Start" menu icon on the lower left corner of the Taskbar and navigate the menus to "System Tools --> GSmartControl". Within the GSmartControl app, double-click on the iMac's hard drive icon to access the drive's health report. The complete health report can be seen by clicking the "View now" or "Save Report" buttons. Post the complete report here.

It is unfortunate that Apple makes this process so difficult.

If one of your other computers is also compatible with macOS 10.11 El Capitan (does not need be using it at this time), then you create a bootable macOS 10.11 USB installer using the instructions in this Apple article:

Create a bootable installer for macOS - Apple Support

The other computer generally needs to be 2008 to 2015 model (some 2007 models may qualify as well) in order to create the bootable macOS 10.11 USB installer. You can use the information in this article to confirm which exact model Macs are compatible with macOS 10.11:

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

Reset the iMac’s password using the following directions.

Change or reset the password of a macOS user account in macOS Mojave or earlier - Apple Support

Tuffy Nicolas wrote:

What else can I do? Surely there must be SOME way to wipe clean a Mac and reinstall the OS with a new user account if you have forgotten the password.

Then you need to erase the whole physical drive if installing macOS 10.11, otherwise you will need to partition & format the whole physical drive.

If you are booting a macOS 10.11 installer, then you need to select the physical drive (or Fusion Drive) and erase it as GUID partition and MacOS Extended (Journaled).

If you are booting a macOS 10.6 to 10.10 installer, then you need to partition & format the drive using the instructions in this article:

https://eshop.macsales.com/tech_center/formatting/Mac_Formatting_6-10_R3.pdf

After successfully preparing the drive using Disk Utility, you need to quit Disk Utility and select the "Reinstall macOS" option (very important if you are booting from local Recovery Mode).

Edit: IIRC, if you boot to Recovery Mode or from the OS X 10.6 Snow Leopard DVD installer, there is a "Reset Password" option on one of the menus of the installer. I don't know if it is still there if booting into macOS 10.11 though.

See if this Apple document will help: If you forgot your Mac login password

Tuffy Nicolas wrote:

But I did erase the iMac's "Macintosh HD" volume and then its entire physical drive, immediately prior to creating the bootable USB stick.

You missed that in the summary which I did quote which is why I mentioned it.

I also, in between those two steps, changed the date of the iMac to 2017, as previously recommended.

Yes, that for when you were using the installer from local recovery mode which used an older outdated version where the certificate had expired. By now using the USB installer which you just acquired, you may need to use the correct date again although I believe another user may have posted on these forums that they had to set the date to 2020 (we don't always get all the details from users and Apple seems to make this much more difficult than it should be which does not help).

But after erasing the entire physical drive, there's no longer any way to boot into Recovery mode -- so how can I possibly change the date on the iMac now?

You still have access to the Terminal app regardless of which installer you use. The only difference between the installers is how you access them and how old/new they are. The Terminal app can be found on the Utilities menu while booted to the installer.

Prior to all this reformatting, the iMac was functioning perfectly, with no problems, and I ever ran Disk Utility on it before all these steps, and it reported zero problems. So I really don't think the drive is failing.

Sometimes reinstalling the OS will reveal hardware issues that may not have been apparent beforehand. Installing an OS does a lot of reading & writing to the drive, plus you may be using portions of the drive that you were not accessing previously or a well worn section of the drive. Even if the hard drive was replaced at some point, most likely even a replacement drive is worn out or failing. I would be very surprised if the original drive is healthy.

However, if the error occurs just before the copying of system files proceeds, then my guess is the computer's date is not compatible for this installer which is why I suggested changing the date back to 2022 or if 2022 does not work, then try 2021 or 2020. Apple's installer error messages don't always make any sort of sense so it is not easy to figure out the problem.

It is possible the USB stick is bad. Try using another USB stick since the quality of USB sticks are extremely poor.

If the error occurs later in the installation process, then a bad drive (or other hardware issue is more likely especially after the copy stage finishes and the installer reboots the Mac to finish installation). For some reason when I read the earlier reply I thought the error occurred later, but reading it again now it appears the error appears almost immediately after selecting the "Install macOS" option and the copying stage never really begins.

The next move, is to Contact - Official Apple Support for assistants.