Yosemite on external drive will not launch past password entry.

I downloaded Yosemite legitimately today. WHen I attempted to logon, I could not, because although there was an account there, a password was being asked for and I hadn't created one.

That should not have occurred. When booting from the external HD for the first time, you should have been greeted with the "Hello" language selection screen, followed by some other steps, followed creating a User Account. So, something went wrong, and it's anyone's guess what that may have been.

One clue may be "I downloaded Yosemite legitimately today." How to download and install macOS - Apple Support provides correct instructions. Be sure to use Safari since the Yosemite link in that document may not load with other browsers. Clicking that link will download the installer app to ~/Downloads.

Erase that external hard disk drive using Disk Utility, expand the InstallMacOSX.dmg file, launch the Install OS X Yosemite app (looks like below), then point the installer at it when prompted.

It should work.

Summarizing the above, follow these instructions: How to reinstall macOS - Apple Support, scroll down to Other methods for older macOS versions, and then Download via web browser.

If that is what you did, and that's what did not work, the fallback would be to boot Recovery with the key chord that corresponds to the following excerpt from How to reinstall macOS - Apple Support:

• If you used Shift-Option-Command-R to start up from Internet Recovery, you might get the macOS that came with your Mac, or the closest version still available.

That should be Yosemite, for your mid-2015 Mac. The weasel word "might" is significant. I prefer "should" but lots of people do lots of strange things to Macs that they should not, so perhaps that's the reason Apple chose "might" instead.

Be sure to select the previously erased external HD for its destination. The installer will refuse to install Yosemite over High Sierra anyway, but don't tempt fate.

After spending way too much time with AI ...

I prefer "wasting".

John Howarth wrote:

HWTech: I installed from Terminal to the same external drive I had previously used in this mission, but the end result was identical to what I already had, ie the login page accepted my password but then presented me with the same login page. It was as if the whole installation process had not happened at all.

FYI, sometimes when installing a second copy of macOS, the system will reboot to the main default internal OS instead of the new copy that was being installed. This usually happens when the installer reboots the system after finishing the first phase which just copies files to the destination volume.

When the installer reboots the system to start phase 2, for some reason it boots to the original default Startup Disk volume. You can reboot the Mac and select the partial installation with an Option Boot. This will allow the installer to finish.....usually the new OS installation becomes the default Startup Disk at that point.

However, I'm not sure whether you actually ran the macOS installer or just used the Terminal command line to create the bootable USB installer.

John: I tried booting into Recovery (command-R), but that took me to a High Sierra download, which could only be placed onto my Mac internal drive.

Correct. That is to be expected, since that Mac already has HS installed on its boot drive.

I then tried an internet recovery (Option-shift-command-R) which led me to a Yosemite download process.

That's the way to do it, but the a couple fundamental prerequisites apply.

The first is that we cannot install OS X on any boot device that already has a superseding OS X version installed. Attempting to do that will always result in the "OSX cannot be installed on this disk" message.

The right way to get Yosemite installed on that Mac is to boot Internet Recovery as you did, and point the installer at a bootable device that has previously been erased. It also has to use the GUID Partition Map scheme and one of the Mac OS X Extended (Journaled) formats. That rules out installing it on the internal drive, unless of course you are willing to erase it first.

The bottom line is that it has to be installed while using a Mac booted into Recovery a Mac running an earlier operating system.

As far as installing Yosemite on an external hard disk drive, it should work. I'm not 100% sure if that's what you attempted though. And as far as installing OS X on a USB flash drive, yes that should work, but having tried it myself several times over several years with several different flash drives I concluded it's an exercise in frustration. They were always either too slow or too unreliable to be useful and I gave up.

That is not to be confused with creating a "bootable USB installer" though as HWTech explained. That technique is typically used to create an installer for mass deployment so you don't have to download it for every single Mac.

This is outdated but it might useful for reference: Upgrading macOS without fear - Apple Community. It refers to Big Sur and some of its links won't be applicable, but the idea is the same.

Was the Mac asking you for a login password? Or for a firmware one?

Set a firmware password on your Mac - Apple Support

"When you set a firmware password on your Intel-based Mac, users who don't have the password can't start up from macOS Recovery or any disk other than the designated startup disk."

In addition to the great advice already provided here.....

You can create a bootable USB installer for Yosemite as well. You will need to follow the instructions for El Capitan and modify the Terminal command line to reference Yosemite. Here is the Terminal command you would need to use which unfortunately Apple removed years ago from the following Apple article:

sudo  /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia  --volume  /Volumes/MyVolume  --applicationpath  /Applications/Install\ OS\ X\ Yosemite.app

Create a bootable installer for macOS - Apple Support

When asked for a password twice, that typically is due to the macOS user account and FileVault being out of sync. The first password is usually to unlock FileVault, and the second one to unlock the macOS user account. It is possible to resync the macOS login password with the FileVault password so only one login is necessary.

It is also possible the system firmware is asking for the FileVault password for the internal SSD (this was added in macOS 12.x Monterey IIRC). In this case, you would need to use an admin login for the internal SSD instead of one for the external SSD you are booting.

John: Thanks for that detailed explanation. I now get the difference between HWTech's method of creating a USB stick as an installer or creating a boot drive. To recap, what I am attempting to do is create a bootable Yosemite drive on an external disk in order to use legacy software for archived projects, which I would run from my Mac which currently has High Sierra installed, but which I want to upgrade as as far as I can after ensuring I have the Yosemite drive available.

However, having checked that my erasure had in fact created a GUID partitioned Mac OS Extended (Journaled) drive on the USB stick, the installer (now on my main Mac drive) still refuses to install Yosemite there, saying it cannot install on this drive. This was a test on a readily available drive, but I also attempted on another external HDD, but that one was not erased before the installation attempts and similarly failed to accept an installation.

But, and I can't believe I'm writing this, in an effort to find another external drive (I have about 30 of them on various shelves) which I could erase in order to make a new installation, I found one on which I had successfully installed Yosemite about a year ago, and which works. I have no memory of doing this, but in the process of searching I also found my own post from over a year ago on the Apple discussions forum, to which you had replied. The conclusion of that discussion was similar to our current conversation, where I had no success, but somewhere along the line afterwards I must have somehow succeeded.

So, to sum up, I have what I set out to do. I don't now how, and I am unable to repeat it, but it appears I can now do what I want to do. Thank you for your help. I've had a frustrating week, but your contributions have eased that frustration somewhat.

John Howarth wrote:

However, having checked that my erasure had in fact created a GUID partitioned Mac OS Extended (Journaled) drive on the USB stick, the installer (now on my main Mac drive) still refuses to install Yosemite there, saying it cannot install on this drive. This was a test on a readily available drive, but I also attempted on another external HDD, but that one was not erased before the installation attempts and similarly failed to accept an installation.

A USB stick is not appropriate for a full installation of macOS. Most USB sticks are both extremely slow and of extremely poor quality. USB sticks are fine for use as a bootable OS installer disk or for transferring small amounts of data, but not as a full time boot drive.

Also, the Terminal command I provided will only create a bootable USB installer drive. You must then boot from that USB installer in order to actually run the installer & install to yet another external drive (either a Hard Drive or an SSD).

Apologies for the delay in responding - I have spent several hours and days following all the advice you have given here, but unfortunately with no positive result.

HWTech: I installed from Terminal to the same external drive I had previously used in this mission, but the end result was identical to what I already had, ie the login page accepted my password but then presented me with the same login page. It was as if the whole installation process had not happened at all.

John: I tried booting into Recovery (command-R), but that took me to a High Sierra download, which could only be placed onto my Mac internal drive. I then tried an internet recovery (Option-shift-command-R) which led me to a Yosemite download process. I pointed that to an erased USB stick, where the installer was placed, but when I tried to actually install, the only HD which was allowed was the Mac internal drive. The USB stick, and also the other external drive I had previously installed to had the message "OSX cannot be installed on this disk". I also tried copying a previously downloaded copy of the installer to the re-erased USB stick, with the same result.

So I could, maybe, have re-installed Yosemite over my High Sierra on the Mac internal drive, but I could not create an external bootable drive, which is what I am trying to do. It does seem that everything is conspiring to prevent me doing that. But thanks all for your attempts to get me on the road to that, even though the road appears to be blocked.