Uniting split files into 1

Hope someone can help.
I have split a large file into pieces using GSplit.
I copied the pieces into the folder on my 2tb external drive from a SD card, the external drive has loads of space 450gb and FAT32 formatted.
The size of the original files is 13.4gb. Each piece is 987 mb. And there are 14 of them.
However, when I try to unite them in the same folder I copied the pieces to, I receive the ‘error 112 not enough space’.
Now my understanding of the product is that it is a solution to the 4gb ceiling on copying files in windows to FAT32 formatted drives. IE. Split the large file, copy the pieces to the destination and then unite them there.
Only this does not seem to work for me. I get the space error when uniting them.
What am I doing wrong?
Is my understanding the product incorrect?
Hope someone can help.

Do you have multiple partitions on your disk? If so, could you please check if the partition where you are trying to combine the pieces has sufficient space available? Insufficient storage can sometimes cause issues with file operations.