Prohibitory Symbol when attempting to downgrade to High Sierra

Nisken

macrumors newbie
Original poster
Mar 1, 2019
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So, I followed Foxlets guide to installing Mojave into my 2009 MacBook Pro as an experiment. However, the OS ran slow and various features were buggy. Therefore I proceeded to install a cop of macOS High Sierra via USB with no luck. I've also attempted to install High Sierra directly onto my MBP's SSD, with no luck. I always get the prohibitory symbol (gray circle with a slash) regardless of install method. The only way i have been able to bypass this is to use an old Snow Leopard disc which eventually reboots automatically and returns me to zero. Lastly, I cannot even enter the macOS utility screen.

When I HAVE been able to get to an install screen the installation of High Sierra tends to fail 2 minutes from completion.

I'm guessing there's some bios level setting that needs to be changed...if that's even possible.

I've searched the forum and no one else seems to be having this problem. My guess is i need to get into terminal somehow to flip some switches.
 

theapplehead

macrumors 6502
Dec 17, 2018
488
504
Chapel Hill, North Carolina
So, I followed Foxlets guide to installing Mojave into my 2009 MacBook Pro as an experiment. However, the OS ran slow and various features were buggy. Therefore I proceeded to install a cop of macOS High Sierra via USB with no luck. I've also attempted to install High Sierra directly onto my MBP's SSD, with no luck. I always get the prohibitory symbol (gray circle with a slash) regardless of install method. The only way i have been able to bypass this is to use an old Snow Leopard disc which eventually reboots automatically and returns me to zero. Lastly, I cannot even enter the macOS utility screen.

When I HAVE been able to get to an install screen the installation of High Sierra tends to fail 2 minutes from completion.

I'm guessing there's some bios level setting that needs to be changed...if that's even possible.

I've searched the forum and no one else seems to be having this problem. My guess is i need to get into terminal somehow to flip some switches.
Unfortunately it is impossible to downgrade your MacOS. Once it has been updated, it cannot be reverted unless it is erased first. That is the only way you will be able to reinstall High Sierra or anything older to your system. Let me know if I am understanding your issue correctly or if there is something I am missing. Hope this helps!
 
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Nisken

macrumors newbie
Original poster
Mar 1, 2019
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Unfortunately it is impossible to downgrade your MacOS. Once it has been updated, it cannot be reverted unless it is erased first. That is the only way you will be able to reinstall High Sierra or anything older to your system. Let me know if I am understanding your issue correctly or if there is something I am missing. Hope this helps!
Thanks for the reply!

The SSD that once had Mojave on it was totally wiped before attempting to use a bootable USB to install high Sierra. Attempts to boot to the USB were met with the prohibitory symbol. I even installed High sierra directly onto the SSD and still received the prohibitory symbol when it was reinstalled. That’s where I’m at now.
 

chabig

macrumors 603
Sep 6, 2002
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3,272
I even installed High sierra directly onto the SSD and still received the prohibitory symbol when it was reinstalled
Huh? Did you install it or not?

To successfully reinstall from your USB installer, boot to it, then open Disk Utility and reformat the drive with a single HFS+ partition. Then let the installer run.

By the way, Mojave isn't really "slow and buggy". It runs fine on supported machines. It would have been more correct to simply say that you had problems with it on your older hardware.
 

Nisken

macrumors newbie
Original poster
Mar 1, 2019
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Huh? Did you install it or not?

To successfully reinstall from your USB installer, boot to it, then open Disk Utility and reformat the drive with a single HFS+ partition. Then let the installer run.

By the way, Mojave isn't really "slow and buggy". It runs fine on supported machines. It would have been more correct to simply say that you had problems with it on your older hardware.
It was installed. The MacBook will not boot to the USB or the SSD with High Sierra installed. The USB bootable was created successfully per terminal.

Slow and buggy given my hardware. I’m sure it’s great on officially supported hardware.
 

chabig

macrumors 603
Sep 6, 2002
6,034
3,272
The MacBook will not boot to the USB or the SSD with High Sierra installed.
Sorry. I'm still confused. I understand it won't boot to the internal hard drive, but you are also saying that it won't boot to your USB installer? If that's right, then you might need to borrow a friend's Mac or have someone you know make a new bootable USB flash drive or hard drive. I think the odds are that yours might be bad. The machine should boot unless it has a hardware problem.

Here is Apple's support document about boot symbols: https://support.apple.com/en-us/HT204156

The prohibitory symbol means that "the selected startup disk contains a Mac operating system, but it's not a macOS that your Mac can use."

Also, try resetting the Parameter RAM (PRAM): https://support.apple.com/en-us/HT204063
 
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Nisken

macrumors newbie
Original poster
Mar 1, 2019
6
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Sorry. I'm still confused. I understand it won't boot to the internal hard drive, but you are also saying that it won't boot to your USB installer? If that's right, then you might need to borrow a friend's Mac or have someone you know make a new bootable USB flash drive or hard drive. I think the odds are that yours might be bad. The machine should boot unless it has a hardware problem.

Here is Apple's support document about boot symbols: https://support.apple.com/en-us/HT204156

The prohibitory symbol means that "the selected startup disk contains a Mac operating system, but it's not a macOS that your Mac can use."

Also, try resetting the Parameter RAM (PRAM): https://support.apple.com/en-us/HT204063
You’re starting to see my issue! Hah

Yes, I used our 2010 iMac running 10.13 to make a bootable High Sierra USB. I’ve tested to make sure it works on the iMac. Put the same USB into the 2009 MBP and get the symbol as soon as I select the drive.

I know the symbol means a prohibited OS which makes me think there’s something lingering on my MBP looking for 10.14.

It SHOULD just boot into the USB but it won’t. Very odd.
 

theapplehead

macrumors 6502
Dec 17, 2018
488
504
Chapel Hill, North Carolina
You’re starting to see my issue! Hah

Yes, I used our 2010 iMac running 10.13 to make a bootable High Sierra USB. I’ve tested to make sure it works on the iMac. Put the same USB into the 2009 MBP and get the symbol as soon as I select the drive.

I know the symbol means a prohibited OS which makes me think there’s something lingering on my MBP looking for 10.14.

It SHOULD just boot into the USB but it won’t. Very odd.
Did you say you cannot use internet recovery mode to reinstall your OS? Try holding Cmd-Option-R when booting your Mac to boot into internet recovery and install the latest MacOS version available for your Mac
 

Nisken

macrumors newbie
Original poster
Mar 1, 2019
6
0
Did you say you cannot use internet recovery mode to reinstall your OS? Try holding Cmd-Option-R when booting your Mac to boot into internet recovery and install the latest MacOS version available for your Mac
Can’t use internet recovery at all either.

Preeeeetty awesome. Hah
 

Nisken

macrumors newbie
Original poster
Mar 1, 2019
6
0
Well, I made a Mojave bootable usb with the same results. I hold down option and select the usb. Directly to the gray circle.

Soooooo, this thing is a friggin brick right now.