How to share hard drive with Tablo and Mac

I bought a 5TB seagate hard drive, as per recommendation, I would like to partition the drive to use some of it as my Mac backup. Every time I go from Tablo to Mac, it needs to be reformatted. I am self taught so I need pretty basic instructions please.

You can’t do that. Get another drive.

For any device (ie computer) to see multiple partitions, it needs to be able to access the MBR (master boot record) to tell said device which partition to use. On a computer, you have that ability to select which partition to boot from. I use to do this all the time on my PC which would have 2 partitions on the hard drive. One parition running Windows the other running Linux. During the boot process I am presented with an option on which partition I want to use.

Tablo does not have this ability. There is nothing in the Tablo boot up process that will allow an end user to select which partition to use. As @FlyingDiver said, get another drive. If you just bought this 5 TB drive you can try to return it can get two 2 TB drives.