A normal shutdown automatically unmounts all disks safely. You can, if you like, go to System Preferences > Startup Disk and select the external as your default. When you restart, it will automatically start from that disk.
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
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