How Do I Move Google Photos to an External Hard Drive?
Copying a Takeout export to an external drive keeps the wrong dates. Fix the metadata first, then copy once, and the drive is a real archive.
Copying is the easy part. The trap is copying a broken library and calling it a backup.
If you drag an unfixed Takeout export onto an external drive, you have archived thousands of photos with today's date and no location, plus a pile of .json files nobody will ever open again. In five years, when you plug that drive in, there will be no way to tell when anything was taken. The sidecars will still be there, technically, but the odds you run the merge then rather than now are not good.
An archive is only useful if each file carries its own story. That means the date, time and GPS living inside the photo, not in a companion file that any copy or sync can separate from it.
So fix first, copy second. Run the app over the export, let it write the real dates and locations into every photo, and optionally have it sort everything into year and month folders as it goes. Then copy the finished folder to the drive. What you put away is a library that any computer, now or in ten years, will read correctly. The first 500 photos are free.