To share with nfs, you need to create /etc/exports on the Pi4 containing just this line:
/media/Passport2 192.168.1.0/24(rw,sync,no_root_squash,no_subtree_check)
Then on the Pi3 either sudo mkdir /mnt/Recordings
and use fstab. I don’t think username and password are necessary. Or don’t create that directory and use autofs where /etc/auto.nfs.shares looks like
/mnt/Recordings -fstype=nfs4 192.168.1.140:/media/Passport2/Recordings
Edit /etc/auto.master as described in the linked post above.
After setting it up systemctl restart nfs-server
on Pi4 and systemctl restart autofs
on Pi3.
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