Disk Is Full Error When Copying Large Folders Onto Empty HDD

I figured it out! It was a pesky icloud issue. Some files within the gigantic 160GB folder I was trying to transfer were not downloaded to my Mac's internal SSD disk, and instead were stored in icloud. When transferring the files, it appears that icloud must first download the files onto the internal SSD, and then once downloaded it will copy to the external HDD. The issue was that I barely had any space left on my Mac's internal SSD (only 52 mb!), so when I tried copying the large folder onto the external HDD, it would fail to download the files stored in icloud because my SSD only had 52mb left.

I suppose when the error message says "The operation can't be completed because the disk is full", Apple is saying that the disk is the SSD, and that icloud failed to download the files from the cloud before the transfer. Very misleading error message since I definitely thought disk was referring to the external HDD!