Reinstalling OS on a Mac Pro with SSD drive

How are you downloading the OS, I hope not from Apple Recovery servers.

You can download High Sierra from here,

open Safari, do not use Firefox,

How to upgrade to macOS High Sierra - Apple Support

Go to Section 4 and click no Get macOS High Sierra

this will redirect you to the App Store High Sierra download page,

it should say Get or Download.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS High Sierra.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

Once the download has completed you will have Install macOS High Sierra.app in your Applications folder.

To make a bootable USB installer follow the instructions in this article.

How to create a bootable installer for macOS – Apple Support

As part of the installation process an installer window will ask you to select a destination disk

for the installation, this is where you can select your SSD drive.

Select the USB drive, press return, the mac will now boot to the USB installer.

When booted up to the installer you will see a Utilities window,

click on Disk Utility press Continue.

With Disk Utility open click on View in the menubar and select Show All Devices.

Select the uppermost disk for the SSD it will probably be named Samsung something or other

click Erase.

give the Disk a name

Format: APFS.

Scheme: GUID Partition Map

click Erase.

If Disk Utility does not give you the option to Format as APFS then select Mac OS Extended (Journaled)

When the reformat has completed then quit Disk Utility.

Select Install OS press Continue to start the installation.

Follow the on screen prompts.

Pressing the Return key will not work you need to press the Continue button.

Do you have a USB wired mouse to hand.