Correct, if you
boot
from the SSD OS X and ArchiCAD will run from the SSD as long as
1. OS X is on the boot drive and this is the drive you are booting from
2. ArchiCAD and it's preferences are stored on the boot drive (SSD)
3. Your project and any links / references are on the SSD as well.
Your iMac will see your external drive as your System boot disk and your internal drive as a SATA connected device when you boot via the external Thunderbolt drive.
Ps. it appears there were two 2010 iMac releases, Mid 2010 and Late 2010, late 2010 had the thunderbolt port....though I can't find any technical specs on it!:)
2012 13" Macbook Pro 8GB Ram, OS X 10.14.6
2010 Mac Pro 2x 6 Core 2.93Ghz Xeon, 48Gb Ram, OS X 10.14.6, RX 580
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