Assuming the files have same names it should be relatively easy: just put the photos into the location where you want them to stay, then open your project, pick any photo and consolidate the photo (via the context menu in the photo panel; it’s important to consolidate the single photo, not the whole item): this will prompt you to select the file from its new location. If you select a file with the same name as the original one, Tropy will assume the file was moved and will ask whether you want to search for other missing files automatically.
In most cases this will find all the missing photos. If your files are scattered across many folders and the structure of those folders is different now than it used to be on your external drive, you will have to consolidate one photo in each set of photos where the folder structure changed.
If the photos are named differently or if they are placed in a very deep folder structure that is different from the one on the external drive, you can use the find-photos script described here: this script will search for all photos in a given folder, compute a checksum and link them to matching photos in your project file.