family photos to flashdrive

Alternatively - if you want to save them exactly as your father in law imported them into the app, you can export unmodified originals.

However, these would not have any edits your FIL applied, and depending on what sort of camera he used might also be in less convenient formats such as RAW files that need a comaptible app to view them. However, if he did shoot raw, this is what he would have probably considered to be his "digital negative". If there are any photographers (or anyone keen on editing his images) amongst his family (maybe even future family) then they would almost certainly value having these files also.

If it were me doing this, I would export both the "unmodified originals" (Exactly as taken) to one folder structure, as well as a normal "export" which will export including any edits, in jpg format (100% quality full size as suggested by yer_man) to a separate folder. I would also be ensuring there are backups of both sets available.

file>export>Export nn photos

or

file>export>export unmodified original for nn photos

(where nn is the number you have selected)

Another hint - both exports have the option of subfolder format. If you choose "moment name", then the files will be put in subfolders based on the photo date (and location if it exists). Looking a little like the second screenshot.

Finally don't try and export everything in one step, the app is likely to run into memory problems if you do.

Instead, select a subset of a few hundred to a maximum of about a 1000 (depending on the capability of the mac) at a time. I would use a smart folder for this, and select by date range - depending on how many pictures he took - of a few months or a year. The smart album definition would look something like this (for example a 6 month period):

And regarding backup - I wouldn't trust the library copy to be a reliable long term backup. It is relatively fragile, only readable on a mac with photos, and is also subject to becoming obsolete over time if apple change the app they use to manage photos (as they have a habit of doing every few years).

File -> Export, and export to Folders on the desktop. Then copy those to the drives.

One major warning: you mention "uncompressed files". In Photos world that means .tiff files which are huge. Are you sure that's what you want? What format are these images now? If they are jpeg them saving as maximum quality and full size is a very good compromise. These are options in the export dialogue.

Tony nailed it.

Export Unmodified originals with "Export IPTC as XMP" so also any metadata additions your FIL might have added (dates, captions, locations etc) are also exported as sidecar .xmp files. No generation loss (pun intended).

Export as JPEG with the default "High" compression with "Title, Keywords and Caption" and "Location Information" checked so any FIL metadata additions (dates, captions, locations etc) are also exported inside .jpg metadata.