Assuming you are using Microsoft Windows, I have an idea for you. Since I was also looking for this solution and found nothing really working! I did a workaround.
What you can do is, Shrink the partition in your Portable, using Windows management tools (right click "computer" and goto Management, then goto "Disk Management"). To shrink the partition, click a partition that you want to shrink, and click on Shrink. Enter the size that is enough to copy all the data, then continue.
When you are done with creating another partition, move the data to the new partition, which is pretty time consuming I get that. What I did, is left my laptop for hours to get this done! Caution: Portables are not supposed to run for hours. I have reliable Portable so I did it.
Here you have two solutions! One to simply format using the right clicking on partition and click on format which will work on external hard drives only. However, I am also giving following solution for someone who may want to format the internal partition to exFat. Windows do not allow internal partitions to be formatted in ExFat by default so we need to follow Disk Part approach!
Note that, if you want to move large files to Portable or Internal Drive, set the Allocation Size to 4096 Bytes, as in the case of NTFS. When you are handling Diskpart, Unit=4096 is doing the same.
Follow as under:
Select Disk x Note: x is the disk number you want to deal with.
select partition x Note: x is the partition number you want to format to exFat
Format FS=ExFat UNIT=4096 Quick
7: You are done!