Partitioning a drive with existing data

You can split the partition, but it is risky and a possibility you could lose your data if something goes wrong. If this is your backup drive, then I do not recommend splitting the partition and risking the backup. A system backup should be a dedicated drive.

Make sure you have a backup of this drive before you start modifying the partitions. If this external drive contains important & unique data, then you should be already be backing up this drive frequently and regularly as well as your main system drive.

https://support.apple.com/guide/disk-utility/partition-a-physical-disk-dskutl14027/mac

genikifor wrote:

Also one other thing i wish to know is... i wish to install/make a bootable partition out of the 80GB partition and install there HighSierra. That partition will be file formatted as APFS, the other partition will be MacOS Ext, will there be a compatibility issue between those two?

In theory no, but I've never done this before.

Will i be able to see -when im on HighSierra- the 820GB partition?

Yes.

And viuce versa, will i be able to see the 80GB partition when i will boot from ElCapitain?

No. El Capitan does not know anything about the APFS file system.

Seems Apple's documentation writers do not know Apple's products. You need to select the physical drive in Disk Utility in order to modify the partitions. Within Disk Utility you need to click on "View" and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility.

What file system is being used on the external drive? If it is NTFS, then you may not be able to resize the partition.