Formatting external drive multiplatform compatible filesystem from macOS High Sierra

Make second partition on the drive using Disk Utility and as long as you don't install to that partition, it will not get erased by an OS (re)install.

You should be able to live shrink your existing partition and create this new space at the end if the partition view of the disk.

I'm assuming you have nothing on the new disk and essentially want to mirror what is on the old disk to the new disk.

You can use the following command line, substituting the proper names for Source-Disk (old) and Destination-Disk (new).

rsync -xavH /Volumes/Source-Disk/ /Volumes/Destination-Disk/

Note: The slash at the end of each path has significance, so don't forget to include it when changing the name of Source-Disk (old) and Destination-Disk (new). Also, if there are spaces in them, make sure you either escape the spaces with a backslash \ or quote the pathnames, but not both.

Explanation of the options used:

-x, --one-file-system    don't cross filesystem boundaries
-a, --archive    archive mode; same as -rlptgoD (no -H)

    -r, --recursive    recurse into directories
    -l, --links    copy symlinks as symlinks
    -p, --perms    preserve permissions
    -t, --times    preserve times
    -g, --group    preserve group
    -o, --owner    preserve owner (super-user only)
    -D    same as --devices --specials
        --devices    preserve device files (super-user only)
        --specials    preserve special files

-v, --verbose    increase verbosity
-H, --hard-links    preserve hard links

Note: Because symlinks are being copied to the Destination-Disk volume the volume name of the Destination-Disk will need to be renamed identical to the Source-Disk, afterwards, for the links to files on the disk itself to track. See the SYMBOLIC LINKS section in the rsync man page for additional options that can be used to modify how the symlinks can be treated.