I've changed the mount value fer ad0s5 in my /etc/fstab from ro (Read-Only) to rw (Read-Write). When I try to copy my screenshot files from 5.3 to the /backup (ad0s5) slice, there's no problem whatsoever. Great. It's now safe to automount it with Read-Write value on every boot. Here's the line:
/dev/ad0s5 /backup rw 0 0
I'm still searchin fer zgv's info. Later.