OS Sierra on an external drive

So i take it you have downloaded InstallOS.dmg, opened it too InstallOS.pkg and then processed that to get the Install macOS Sierra.app, in your Application folder.

Ok so when you start the installation and go through the first couple of installer windows it should allow you to choose which

disk you want to install, sometimes you may see one disk with a show all disk button, or sometimes you will see the icons of all disks available for installation.

If you are not seeing your external disk as an offered destination then it may be because it is not in suitable format.

In order to install macOS Sierra the disk needs to be formatted as Mac OS Extended (Journaled) on a GUID Partition Map.

If it is not you will need to reformat the disk.

Open Disk Utility in the Applications/ Utilities folder.

In the lefthand panel select the Disk of your external drive

you will find this in the 'external' section at the bottom,

so select the Disk not the indented Volume.

click on Erase

give the disk a name

Format: Mac OS Extended (Journaled)

Scheme: GUID Partition Map

click Erase.

When that completes, quit Disk Utility

start the installation again and see if you

now get to choose your external disk

as a destination for the installation.

If you copied the .dmg to your SSD and ran it from there at the point where it asks you to choose a disk to install it to

it has to be pointed at a disk that has an OS on it because it is looking for an Applications folder where it can place

the Install macOS Sierra.app. This would just leave you in the position you were on before.

Is your SSD formatted as Mac OS Extended (Journaled on a GUID Partition Map if not you will need to do so to install

macOS Sierra.

What you need to do is make a bootable USB stick using the Install macOS Sierra.app and the createinstallmedia command

in Terminal.

Put the Install macOS Sierra.app back into your Applications folder.

Open Terminal (Applications/ Utilities) now copy and paste this command into the Terminal window,

sudo plutil -replace CFBundleShortVersionString -string "12.6.03" /Applications/Install\ macOS\ Sierra.app/Contents/Info.plist

press return

enter your password

press return

Now paste in the createinstallmedia command for Sierra where MyVolume can be replaced with the name of your USB stick

If your USB name has more than one word separated by spaces you would need to type it in as

/The\ USB\ Name

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ Sierra.app

press return 

follow the prompts

When that is done you will be able to boot to the USB stick and install Sierra.

Restart your mac while pressing and holding the option/ alt key.

After a couple of minutes you will see the Startup Manager, select the bootable USB, press Return.

The mac will boo to the USB you should see a Utilities panel, select Install OS, press Continue.

The installer should start up follow the prompts and when asked choose the SSD for the installation.

Have you definitely checked that it is formatted correctly, is the external drive connected directly to the mac and not connected via a USB hub of any kind.

Same frustrating problem here. New SSD, wiped and formatted correctly. Downloaded InstallOS.dmg, opened it and now have the Install macOS Sierra.app. in my Application folder. Here's where it's different for me. I can't open the installer from my internal drive on my Mac Pro (running Mojave) so I copied the .dmg to the SSD and opened it there. It gets all the way to choosing a disk and then tells me I can't install on the external drive because it needs to have Mac OS already installed on it. Any ideas? I really need to use some legacy programs, specifically Final Cut Studio.

Thank you Eau Rouge! This definitely got me to where I needed to be. Got it installed on the SSD and was able to boot the Mac Pro from it. Of course, a 3rd party program I wanted to use in Sierra suddenly won't work there when it should, but I bet that has more to do with Apple security than Sierra os itself. But now I can run my Final Cut 7 with no problems, at all. Thank you, thank you, thank you!