Back up data from external hard drive to iCloud

I’m trying to backup 3.5 TB of data from an NTFS formatted external hard drive to my iCloud account. I’m using a rented MacBook Pro 2012 with 500 GB drive (best I can rent considering where I live).
All went well for the first while, but then I got an error message saying my disk is almost full (see screenshots). The About Mac function shows that there is 488 GB of ‘Other’ data. I’m assuming that’s either local copies or cached copies of what I’m uploading to iCloud?
Where can I find this data to free up some space?