MBA 13 2017 High Sierra. External LaCie 1TB SSD Time machine added, partition grayed out
The Disk Utility application can partition the external drive, if you use GUID Partition Map scheme. Also, you need to create a one or more temporary Mac OS Extended (Journaled) partitions in order keep the partition button from being grayed out.

Below is an example of how to create two MS-DOS (FAT) partitions using a 16 GB flash drive.


Erase the drive.


Partition the drive.


Erase the first partition.




The results are shown below.