Receiving error when copying files to external APFS drive

I'm trying to copy a film project from external drive A (formatted with ExFAT) to external drive B (formatted with APFS). This always results in an error message saying: "the item "name" can't be copied because there isn't enough free space". However, I have now formatted the external drive B multiple times and it's completely empty. I am assuming that this is happening as I'm trying to copy files from ExFAT to APFS. If this is the case, how can I then do that?