Hi,
I'm Rodrigo and I'll help you.
The THUMB Z drive has 1 partition and it is the using the whole drive.
You have two options: Shrink the existing partition and create a new partition on the unallocated space or delete the current partitions and create two new partitions.
Since you said the shrink option is greyed out, delete the current partition and create two new partitions.
Backup your data, this procedure will delete the files on the drive.
Right-click the blue bar on the disk > Delete volume... > Yes.
Now the whole drive space will be unallocated.
Right-click the black bar on the disk > New simple volume... > Set the partition size > Drive letter > Label and file system > Finish.
Do the same procedure on the rest of the unallocated space (black bar) to create another partition.
To capture the drop-down menus on Snipping Tool, use the shortcut Windows + Shift + S.