Disk management options greyed out for external drive

That has been a UEFI system disk I guess - if you no longer want it as a system disk, then best use a 3rd party partition manager e.g. Minitool Partition Wizard. There will be a small partition Disk Management doesn't show.

Is there anything you want to keep? If not, delete all the partitions and reformat.

H: looks like it's probably a Factory Reset partition- to restore the PC to 'as bought'.

F: - presumably was a Windows partition ( 'C: ' ) - depends if you've been using it for something else.

As @dalchina mentioned, the drive is a UEFI-GPT system drive.
If you don't use it anymore as a boot able drive I suggest you make a backup of the data on it and then clean and initialize it with Diskpart. ALL DATA ON IT WILL BE DELETED

Open a CMD window as administrator and type:

diskpart
list disk (it will list all drives. Identify the USB drive number-Should be 2)
select disk n (replace n by the USB drive number obtained with list disk)
clean
create part primary
select part 1
format fs=ntfs quick
assign
exit (to exit diskpart)

Just to be safe turn pc off remove power from this drive start pc does it boot ok just to check the system files are not in use

Another method I use for fully cleaning a drive [usually in a drive dock] is GPARTED [GNU Partition Editor], available as download .iso file used to create a Bootable CD or as a program in a Bootable Linux LiveDVD or installed on a Linux computer. It can remove little partitions that Windows sometimes can't. But then I probably do it more often when salvaging parts from otherwise junkable machines.