What is with the purgeable space on my USB drives?

I have several USB drives which I use for transferring media from one location or device to another. They are formatted FAT32 or NTFS by necessity. Since I acquired a replacement MBP17 with High Sierra, my non-Mac formatted drives are somehow enslaved to Apple's tyranny.

As an example, I have a 16GB drive which says that it has 6GB free but when I try to copy a 4GB file, I am told that there is not enough space to do so as a further 2GB is required and I am given the option to 'Manage Disk Space'. When I click this option, I see nothing relevant to the USB drive whatsoever.

When I 'Get Info' on the drive, I see that it says that it has 2GB free and 4GB which us purgeable.

On searching other people's thoughts and experience, I discover that the purgeable space should be purged when it is required, well guess what!

Apparently, this has something to do with iCloud Drive but what does the storage space on an external USB device which is not even formatted to an Apple format and in many cases is not even my property have to do with iCloud Drive. And if the space is supposed to be purgeable, why can't it be purged.

I'll be frank, I've been using Macs for 34 years, I'm getting close to retirement and until a few years ago, I couldn't imagine not having my Macs to work with but recently, stuff like this just wears me down and I look forward to the day when I just get my creaky old MBP out once a week to check my e-mail.