Let's assume that you are on a Mac (I assume from the shortcut cmnd+I).

-> general info which is applicable to all operating systems
1. this HD is a mechanical disk, so large backups (e.g. 100 gB) will take some time to add up all the file sizes when you use cmnd+I. Install a tool (I recommend menu meters) that shows you if your HD is working or not. Then you know if it is still calculating or already done and you know if the problem is with the software or the user ;-)

-> continue with Mac Infos
2. display the folder size in the window of your backup. Then you can compare the different subfolders with each other if you have two windows open side by side. One window with the ext. HD & one from the source volume. Be sure you are in the Finder window in "List View" (cmd+2). Then call up the "Show View Options" (cmd+j) & activate "Show Size" & "Calculate All Sizes". The latter will sum up all files in the subdirectory, otherwise it will only show a dash for the folder size. The summing up can take a while but at least you see a progress, because little by little each folder shows the summed GB size.

Use the "rsync -av <SOURCE> <DESTINATION>" command in the terminal window to copy your files (press cmd+spacebar & type terminal). Replace Source & Destination with the folder you want to copy & Destination with the ext HD by dragging them into Terminal. Terminal will then write the correct path name.
Advantages: Copying with the terminal shows you file by file which was copied, you can save the log file & it also shows you error messages at the end of the copying process with a list of files that caused problems. Then you can decide if you want to copy the files again (by hand) or if you want to abandon them to digital nirvana. By the way: "rsync -av" resumes aborted copy operations (even after a crash) & it continues even if a file is corrupted instead of cancelling the copy operation. You can even run the command again after a successful copy to make sure that everything has been copied (arrow key up, show last command in terminal again & press enter). It will synchronise everything in no time & show you error messages or success. If you would like to see other benefits & instructions on how to use it, I recommend the following link: https://blog-it-solutions.de/mac-os-bac...

4. use a backup software that does incremental backups, i.e. only changed & new files are backed up. Timemachine (on your Mac) does this & if you want a bit more control you can use carbon copy cloner or another tool.

Have fun shovelling data.
Hoesu

Unfortunately, we can't answer this question for you, as we haven't been able to try this out in such a form, but I'll be happy to enter it in the community, perhaps a user can help you :)