Installer flash drive won't show up in Start Up Disk Preference

You do not need any specific version of the macOS installer here. Assuming you created the bootable macOS 12.6.5 USB installer correctly, it should work. However, maybe the USB stick used is bad or not compatible with booting a Mac. Macs can be very particular about the drives used for booting. Plus the quality of many USB sticks is extremely poor.

Verify you created the bootable macOS USB installer using the instructions in this Apple article:

Create a bootable installer for macOS - Apple Support

If you followed these instructions, but are unable to see the USB stick as a bootable drive, then try using another USB stick, especially a different brand.

There is also a possibility the issues you are trying to fix with a clean install are the same issues preventing you from seeing a bootable installer on the Option Boot screen. I do know that some hard drive failures can be severe enough that they cause the Mac to hang trying to read the failing drive that the system times out looking for any other bootable media.

You may want to try running the Apple Diagnostics to see if any hardware issues are detected. Unfortunately the diagnostics do not detect most drive failures. If you can still boot the iMac normally (or in Safe Mode), then you can check the health of the drive(s) by using DriveDx...feel free to post the complete DriveDx text report for each drive. If the Mac cannot boot into macOS and the diagnostics don't report any problems, then I can provide instructions for creating & using a Linux boot disk to check the health of the drive(s).

A bootable USB Installer does not show up in Startup Disk. You must hold down the Option key on startup.

If it doesn't show up in the Startup Manager, then it either isn't configured correctly or something isn't recognizing the drive at all.

You don't need to start at 12.0.1.

gregory l. wrote:

The Option boot method just doesn't work for whatever reason (nor has any other 'key combo while starting' worked either, and I've tried Option, D, R, and maybe others.) So if the drive doesn't show up in Startup Disk Preferences I don't know what to do.

You can try booting a Knoppix Linux USB stick, or even a Linux USB installer such as one for Linux Mint. The point is to see whether you can see a bootable volume of any kind when Option Booting the Mac (I'm assuming you see the Apple boot picker menu even if no external bootable volumes are seen).

Use the downloaded Linux .iso file as a source for Etcher (Mac/Windows/Linux) which will create the bootable Linux USB stick.

But I will try another flash drive.

Technically you can also use a regular USB hard drive or SSD if you have one to spare which doesn't have any data on it.

The results of the test are in the "Additional text" feature, as it puts this message length beyond acceptable limits, apparently. I don't know how to access that from your end, but there must be a way.

It will show up in your post ideally as a drop down box with whatever title you created, or it may just show up as a link (the behavior is sometimes hit or miss with the Apple forums for the Additional Text appearing as a drop down box). I don't see either one in your post. Try posting again to see if it shows up the second time.

Here is a test of what the "Additional Text" should look like once posted to the forum:

Edit: Once the whole post is submitted, then you should see the drop down box yourself as you should see it in my post. However, some times it may not show as a drop down box, but instead just a link if you hover the pointer over it.

richard graham iv wrote:

I agree it should show up in startup disks in setup.

That stopped at some point, probably the same time Recovery stopped showing up. It will only show up in the Startup Manager (holding down Option key).

With the flash drive plugged in boot with the Option key held down and see if it shows up in the list of bootable volumes.

gregory l. wrote:

Interesting. How did you boot from it then? Option key upon restart?

Yes, holding the option key is the only way to boot a Mac from a bootable installer. It's always been the case.

For the DriveDx report issue you just need to Copy the contents from the DriveDx text report into the macOS clipboard, then with the "Additional Text" box open on the forums, Paste the contents of the clipboard into the bottom half of the "Additional Text" field. Also make sure to provide a title before Saving the "Additional Text".

If you don't see the Apple boot picker menu, then make sure to use a wired USB keyboard since wireless keyboards may not go ready soon enough to activate the special startup modes. You need to press & hold the Option key immediately after hearing the startup chime. Holding the key too soon may not allow the Mac to register the keypress....more than a second after the startup chime & the system may already be booting.

If no bootable volumes are seen on the Option Boot screen, then you should see a drop down box titled "Choose Network". This is the Apple boot picker menu with the Option Boot. If the WiFi credentials are stored the PRAM, then maybe it would try to automatically boot into Internet Recovery Mode if no other bootable volumes are present. Resetting the PRAM should prevent that and have the system sit at the Option Boot screen. It is also possible to connect a bootable USB drive after reaching the Option Boot screen and have it show up if the drive is actually bootable & working.

Try booting into Internet Recovery Mode (Command + Option + R) at some other physical location if using an Ethernet cable does not help. Trying on another network and with another ISP may be necessary. There may be something with your home network or router configuration, or even ISP which is incompatible with Internet Recovery Mode. Or maybe there is a hardware issue with the iMac which is causing all the problems including being able to boot a macOS installer or accessing the diagnostics.

If you cannot clone the drive or access the installer (USB or online), then you will need to have Apple or an Apple Authorized Service Provider assist you.

Good luck.

Booting is different than just seeing data. Like I said, Macs can be very picky about the drives used for booting. Plus I have found many USB sticks to be of very poor quality even from name brands. Issues with USB sticks can be intermittent. You definitely need to try another USB stick...a different brand is also a good idea just to be safe.

Like @Barney-15E mentions, not all bootable drives will show up in the Startup Disk System Preferences. The Option Boot screen tends to be more reliable for seeing bootable volumes (I think macOS gets in the way of things sometimes).

I don't see the DriveDx report.

The Option boot method just doesn't work for whatever reason (nor has any other 'key combo while starting' worked either, and I've tried Option, D, R, and maybe others.)

I'm guessing Recovery doesn't work, either? You could reinstall from there.

How to reinstall macOS - Apple Support

There are various key combos to get different installs, but the current links to those don't seem to work. Shift-Option-Command-R (I think) will install the OS that shipped on the Mac. Perhaps then you can upgrade to something you want using the Full Installers as noted by dialabrain.

If none of the startup key combos work, I'm afraid there is nothing you can do except take it into an Apple Store.

It has nothing to do with the USB or the macOS version on the drive.

The Startup Manager is what you should get when you hold down Option on restart.

FWIW, the earliest version of Monterey still available is 12.6.2.

You can get it by runnng this command in the Terminal…

softwareupdate --fetch-full-installer --full-installer-version 12.6.2

You're welcome. FWIW, assuming the installer you created is correct there's no reason restarting your iMac and immediately holding the option key until the drive mounts shouldn't work.

dialabrain wrote:

FWIW, because I wasn't sure, I created a Ventura 13.3.1 Bootable Installer and it does not show in Startup Disk either, despite being able to boot from it.

Probably because most people don't want to boot the installer every time.

I've also seen this with some non-Apple boot drives as well.

Well, I can't think of any reason a Monterey 12.6.2 bootable installer wouldn't work, or for that matter a 12.6.5 installer. I was just letting you know 12.0.1 is no longer available.