I'm unable to copy files because of this "Not enough disk space to copy" bug

I am copying a bunch of small files (around 25 MB to 200 MB) to an external hard drive. My MacBook Pro has a 500 GB drive with 179 GB free on it. "Purgeable space" is listed at 6.3 GB. So can somebody explain to me why this thing is refusing to let me copy files, giving me this ridiculous "not enough disk space to copy" warning, and demanding variable amounts of space be freed up (anywhere from 52.4 MB (!!) to a few hundred MB) when it obviously has an incredibly large amount of space to work with. Please don't give me the standard answer for this issue "delete some files" because disk space is not an issue, despite the warning. This has to be a bug.