Moving many files from one Raspberry Pi to another

Nextcloud version 12.0.2:
Operating system and version NextCloudPi (Raspbian GNU/Linux 9 (stretch)):

The issue you are facing:
I have many files to migrate to NextCloudPi to an external USB WD Passport HD.
What is the best way to format and mount the drive? Filesystem?
I have tried the BTRFS with Automount. Updating permissions, installing ftpd and many issues.

Also setup an external storage to the other source system ftp and tried moving over ~800 mb of files… went for a while and then started getting errors about failing to open stream and imagecreatefromstring(): Empty string or invalid image at /var/www/nextcloud/lib/private/legacy/image.php#622

Since this is just for personal use, I’m not so much interested in segregating security for the www, root and pi user. Just want to move the files so that I can access them via ssh or the web ui. Keeping it simple.

Not too much interested in troubleshooting, rather than just knowing steps (even high level) of best approach.

Thanks for your help.