MacOS – Synchronise/Merge MP3 collections without duplication or huge copies

If the photos and measurements are attachments you could use Automator to download the selected messages to a folder.

Edit: I wrote this AppleScript that will download the message and the attachments for all the selected emails in Mail. Make a new folder in your home directory called Emails and run the following code in AppleScript Editor.

NOTE: Make sure the Emails folder is empty. There may be problems if there are items in it already.

tell application "Mail"
    set the_messages to selection
    repeat with this_message in the_messages
        set message_subject to subject of this_message
        set message_body to content of this_message
        set download_path to "~/Emails/\"" & message_subject & "\""
        set save_path to (POSIX path of ("/Users/" & (short user name of (system info)) & "/Emails/" & message_subject & "/"))

        (* create a directory for the message and attachements *)
        do shell script "mkdir -p " & download_path

        (* save message body into a file *)
        do shell script "echo \"" & message_body & "\" > " & download_path & "/message.txt"

        (* save the attachments *)
        repeat with the_attachment in this_message's mail attachments
            save the_attachment in save_path & ":" & (name of the_attachment)
        end repeat
    end repeat
end tell

You could use mdfind in a shell script to simulate your smart folder's settings (mdfind is how you use Spotlight from the command line). Use kMDItemKind to specify the kind and kMDItemAudioBitRate for the bitrate.

Then you can use that list of files and copy them to your external drive.

Something like this script below. (scp copies files to a server; use cp to copy to a drive. Or, if you want to get fancy, rsync is generally better for incremental backups.)

#!/bin/bash

for FILE in $(mdfind 'kMDItemAudioBitRate >= "44000"')
do
    scp $FILE myname@myserver:~/music-backup
done