Seagate branded external drives come preformatted, some for PCs, some for Macs, and some of them have special utilities allowing them to work between both without formatting.
Seagate internal drives must be formatted before use. Drives that will be bootable are formatted during the process of installing the operating system (ie, Windows or MacOS).
Drives that will be used for storage can be formatted according to the instructions below.
Or, if you have a drive that is formatted for a different type of computer or a drive that is not preformatted, you will need to format the drive before you can use it.
Warning! Formatting erases all data on the drive. So if you are formatting a drive that has already been used, be sure to copy any files you will ever want to see again onto a computer or backup drive.
For instructions on how to format your hard drive on a Mac, please reference the following links:
How to format a drive in Mac OS 10.6 to 10.10
How to format your drive in MacOS 10.11 and above
Formatting your drive in Windows
Note: If it does not you will need to right-click and delete the volume first.
If the external hard drive is pre formatted for Mac, Disk Management will list 3 partitions and you will not be able to delete the "Health (EFI System Partition)" because it's a protected partition. To delete this partition you have to use Microsoft Diskpart Erase utility. Please see here the instructions.
See more information at the Microsoft Support Knowledge Base.