Disk doesn't use GUID Partition Table scheme

I have read through many different 'answers' to this on the Support site, and none of them address my issue, so please read and help. I bought a new 2TB SSD external, and I want to install macOS High Sierra on it. (this mid 2010 iMac I'm using won't install anything higher). So, step 1, format the SSD to mac OS Extended Journaled (because it came to me FAT32 MS Dos. ) Done. Step 2, get the High Sierra installer onto my iMac. Done. Step 3, run the installer and try to choose the SSD. It's grayed out, and the reason given is "This disk doesn't use the GUID Partition Table Scheme. Use Disk Utility to change the partition scheme. Select the device containing the disk, click the Erase button, select the Volume Scheme and then click Erase." Well, friends, this works just fine but there is Nothing about a Partition, and the button for partition is grayed out. Tried it anyway. Same result.

Okee dokee, let's use restore. I clicked on my SSD in Disk utility, and clicked Restore. the source is Macintosh HD. The error says "Can only be done on GPT partition maps."

Dead end everywhere. How can I make my new, fast SSD my boot and start-up disk, and let the iMac run from it, instead of from the 2TB SATA hard drive?