Hi Sylvie,
What brand is your external hard drive? There might be some compatibility issues with the macOS High Sierra. And if not so then, your external hard drive file system seems to have become corrupt hence there is no recognizable structure on your drive. Your data is still present on the external hard drive, but due to the corruption of the structure, you won't be able to access it. Please be cautious - don't format your external hard drive until unless you have extracted your data from it. After data restoration, you can navigate to the Disk Utility and select your hard drive for initializing. On your macOS High Sierra, you can format the external hard drive to APFS, HFS, HFS+ however if you plan to use your external hard drive as a Time Machine then let it be HFS or HFS+. Here is a guide which might help you recover your data [https://www.stellarinfo.com/support/kb/index.php/article/recover-data-from-initialized-mac-har-drive]
Helpful read [https://www.macworld.com/article/3240968/storage/watch-out-macos-high-sierra-could-format-your-time-machine-drive-using-apfs-and-thats-not-good.html]