How to make external SSD as your startup disk with your iMac?

> SSD drive showed up as greyed out and when trying to install “use disk utility to change partition scheme” but that is greyed out

Select View > Show All Devices. Then select the device (not any volume under it). Then select erase, set partition scheme as GUID and format as APFS. Then continue installing macOS.

These two user tips describe how to clone your internal drive to an external SSD and make it bootable:

Using Carbon Copy Cloner to create a bootable external SSD of an Intel Mac's internal drive - Apple Community

Boot from external drive on mac - Apple Community

I recommend getting your external SSD from OWC (MacSales.com) as they are considered the premier online hardware supplier for Macs.

First you need to connect it using the USB cable provided by the vendor.

• Next, format it the same as your HD is formatted using Mac's Disk Utility app. Do NOT use any maintenance app that came with the SSD. It will cause problems for Mac users you don't want. Just delete it.
• Next, download the operating system you are using, or want to use if different. They can be found in the Apple Store. You can do a search to access the download link. The download may take up to 45 minutes or so. Use an Ethernet connection and not Wi-Fi. It is too slow for these very large files and mostly causes problems or file corruption.
• Open the downloaded file and select the disk you wish to install it on. Click the installation link.
• The installation may take another 30 to 40 minutes or so with several automatic restarts.
• You will then be guided through the setup process where you will also create your own computer access code and your Apple ID. You may choose to use the same ones you created when you set up your computer. Write them down and save then somewhere secure for future reference in case you forget them (as so many people seem to do here on this forum).
• During the setup process you will be asked if you wish to import data. I will assume that you want to set up the SSD the same as your HD, so click on yes and then follow the instructions for turning on the Migration Assistant app in both disks which does the transfers.
• If you use Migration Assistant you must transfer all or none of the apps and/or files. You cannot select only certain ones.
• At the end of the installation your SSD will have been selected as the startup disk. This can be changed at any time in the System Settings app.
• No files will be deleted from your HD by this process.

Just pay attention and carefully read and follow instructions and everything should go well.

Glad to help and welcome to becoming a Mac user.