How do I encrypt my USB External Flash Drive ? Keeps saying" A GUID Partition Table (GPT) partitioning scheme is required.

If you're being told to use a GUID partition scheme, repartition your flash drive using Disk Utility, and click the Options... button to open the window in which you can choose a GUID partition scheme. I assume you're already giving it a Mac OS Extended (Journaled) format.

Open Applications >> Utilities >> Disk Utility.

Look in the list on the left for your USB drive.

Here you can see the details for my USB Drive:

At the bottom is the Partition Map Scheme, which as you can see is GUID

To change the PMS, you need to reformat.

Choose the Partition Layout drop-down and change it from Current to whatever you want.

Click the Options button at the bottom and select GUID.

This will erase ALL data on your USB. Make a backup first if you need to.

Hit the Erase button to reformat.

Once you have done this, you can format each partition separately. One of your options is Mac OS Extended Encrypted.

You can see that my USB has 2 partitions in the map. You can have between 1 and 16. Unless you have a need, like my 2 different installer partitions, go with one.

Once the Map is complete, you'll see an entry called Untitled. on the left. Select this.

Select the Erase tab.

Change the format drop-down to (Journaled, Encrypted).

Click Erase.

You should be able to create a slide show using a presentation app. Finder would require you to trigger each item.