I have kinda fixed the problem. Once I shut down I have to unplug both the pi and the hdd's power and remount on startup by adding @reboot sudo mount -o remount,rw /partition/identifier /mount/point to sudo crontab -e. This is obviously not ideal but at this point it's good enough for me. Thanks for all the help.
Edit /etc/fstab and add
UUID="46205A22205A196F" /media/pi/Media/ ntfs-3g auto,nofail,noatime,rw,user 0 0
and it should mount automatically at bootup. Bootup won't fail if the drive is not plugged in because of the nofail option.
One step you may have overlooked was checking man fstab - in other words, the system manual for creating & maintaining /etc/fstab. Even though it's a bit dated now (Feb 2015), and has at least one error (re behaviour of nofail), it does reflect the software on your system. I've always found the fstab incantations a bit arcane, and man fstab at least does ...
I'm using a very similar line in /etc/fstab, you can try something similar:
UUID=76634fc7-bf80-470e-9cf7-727e8e13581c /mnt/slave_hdd/ ext4 defaults,errors=remount-ro 0 0
Alternatively, reading logs in /var/log/ may help you.
First flash Raspberry Pi OS to a spare SD Card and boot it in the other RasPi. If it works, poweroff both RasPis, attach the two external USB drives to the other RasPi and boot it up. Now you should be able to mount the ext4 partitions on the attached drives.