#Solution
Many thanks to steve for his simple solution. Of all my searches I did not happen to come across this.
If df, fdisk -l, umount -l, pumount do not work then you should check /etc/mtab next. This file had the following contents:
/dev/sda2 / ext4 rw,errors=remount-ro 0 0
proc /proc proc rw,nodev,noexec,nosuid 0 0
sysfs /sys sysfs rw,nodev,noexec,nosuid 0 0
none /sys/fs/cgroup tmpfs rw,uid=0,gid=0,mode=0755,size=1024 0 0
. . .
systemd /sys/fs/cgroup/systemd cgroup rw,nosuid,noexec,nodev,none,name=systemd 0 0
/dev/sdb1 /media/sdb1 ext4 rw,nodev,nosuid,noexec,errors=remount-ro,user 0 0
That very last line was the source of the issue. Simply removing it fixed everything.
Please try using other guides/solutions before attempting this. I am not aware of any impact this could have on your system or device if other services are actively attempting to read/write/lock this partition.
