mkdir (If it is in a dir where you have no write access, you need to sudo or doas)
Unmount the automounted /dev/sda1
umount /dev/sda1
Then mount sda1 to the newly created dir
mount /dev/sda1
Then you can use genfstab to create a fstab entry. (You maybe need to sudo pacman -S arch-install-scripts)
genfstab /
This will write a fstab file to stdout (the terminal). Look for the line with , copy it and sudo open the /etc/fstab file with your prefered editor. Add the line at tge bottom and add the flags rw,user,noauto to the entry.
This way you have to manually mount sda1 every time you boot with mount /dev/sda1
You can add that to your .bashrc or equivalent. (If you don’t plan to remove the disk, you can skip the noauto and the drive will be loaded automatically, but if it is unplugged your system won’t boot normally). Maybe there is a better way, but this way works for me good enough.