How can I repartition and format a Samsung Portable SSD T1 (model#: MU P8500B)?
You don't need any additional software or drivers, not that I am aware of. You can do everything via disk utility. Please backup your data before performing this (by this I mean actually copying your files onto some other media or the cloud).

Please note: The method will vary for different macOS versions, as in the position and labeling of the icons are possibly different but the functionality should remain the same.

The steps given below are for macOS Sierra. Warning: Wall of text incoming.

Your disk utility interface should look similar to this (except the options on the top bar shouldn't be grayed out):



Next, you need to select the whole disk in the column on the left. As you can see in the image above (look at the column on the left), the volumes are stacked under the actual disks and are indented slightly to the right. You need to select the disk, and not the volumes. Your disk should be labeled "Samsung Portable SSD T1 Media" from what I can see in the given image.

Next, navigate to the bar on the top and click erase. There you should get a confirmation message, for which you should select the option "Erase". This is immediately followed by a window with a field for the name you want to give the disk, and two dropdowns - one for which format you want to use, and one for which scheme you want to use.

Here you need to make a decision. You can name the disk whatever you want, obviously. But as for the format you need to consider your use-case. Are you going to be using this disk only for Macs? Or are you also going to plug it in to Windows and Linux machines? Actually, Linux machines can also read Mac OS extended, but I digress.

If your use-case is the former, then you should just go with the default options it gives you which should be - Format: Mac OS Extended (Journaled) and Scheme: GUID Partition Map. If not, change it to that.

If your use-case is the latter, then you should change the format to exFAT and let the scheme be GUID Partition Map.

You don't really need to change the scheme unless you plan to make the media bootable. If you do, then please tell me the details of the OS.

Finally, you can press the erase button and let the magic happen. If you get an error, then simply click on erase again and it should work. I'm not sure why this happens, and this bug seems to have increased on Sierra. I think it has something to do with USB hubs, but it's not much to be concerned about.

After the erase process has happened you should have just one partition. Now you can paste the data back in and enjoy your drive with a bigger partition!

Good luck