I'm no lawyer, but if the license agreement says that you can only install it in one computer, you don't violate it by installing it in an external drive because the external is not in a computer. Take my legal advice for what it's worth (maybe 2¢?).

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365