back up pictures to external drive?

If you would like to copy the single photos to an external drive you would have to export them first from your photos library and the save them on an external drive.

Export photos, videos, slideshows, and memories from Photos ...Official Apple Support › guide › photos › mac

You would have to create a folder structure to make it easier to find them, 10.000 photos in a single folder can be challenging.

I would recommend that you export not all photos in one run, export only a 1.000 photos in one run.

It will get very slow / create issue if you export a few thousand photos in one run.

Or you could copy the complete photos library as a single file to an external drive.

Move your Photos library to save space on your Mac - Apple ...Official Apple Support › en-us

Prepare your storage device

Make sure that your external storage device, such as a USB drive or Thunderbolt drive, is formatted for Mac: either APFS format or Mac OS Extended (Journaled) format.

You can't store your library on a storage device used for Time Machine backups. And to avoid possible data loss, don't store your library on a removable storage device like an SD card or USB flash drive, or on a device shared over your network or the internet, including over a cloud-based storage service.

If you use this as a backup you will not use this copy as your photos system library!

Ralf

Ralf-F wrote:

...

You would have to create a folder structure to make it easier to find them, 10.000 photos in a single folder can be challenging.

..

Ralf

A suggestion, in addition to your excellent advice: We can let Photos create a folder structure automatically.

I like to export the photos into subfolders named like the moment (location and date). This is working well, if all photos are having locations assigned.

This is particularly convenient, if we change the date format in the System Prefererences > Language and Region to the format Year-Month-Day, so we can sort the moments folders by the date.