Ok might not be related to your issue but you should try to avoid operating a HDD without dedicated PSU, because RPI4 USB ports are not designed to fully power HDD. At the moment this might gonna work in your case but your are running a risk to get data corrupted due to power shortage and voltage drops. Especially during boot this could be the case. We had quite some issues where user lost their entire system this way.
You can watch potential issues be checking for kernel error message
dmesg -l err,crit,alert,emerg
looking to your issue, could you share following
cat /boot/cmdline.txt
cat /etc/fstab
lsblk -o name,fstype,label,size,ro,type,mountpoint,partuuid,uuid