OP that is 1.8 TB of file (s). It is usually the file allocation table used and the piece of software used to prepare the drive. You have plenty of space, but the number of files has exceeded the (FAT, NTFS or what ever is use) ability to TRACK the number of files correctly..
I suggest you break up the number of files to copy in 1/4 blocks. Break up the large drives into 1 TB or so per partitions. Use 2 or 3 quality drives. I don't like to go over 75 % of the drivecap or partitions over a TB.. ALL drives I use are split in at least 2 parts, when set up for BACKUP. A boot drive is different, a storage drive is different, a backup drive is different. Usually different drives..so they are set up different.
A Boot drive I try to use SSD, a storage drive use redundant drives and back ups, may very.. HDs, Tapes, A LOT of CD, Cloud, RtR, Vinyl, how else do they store STUFF... :-)
It is just good practice.. I think if you break up the file NUMBER being copied at one time, That will make a big difference..
Another way is to BIT copy the drive Clone it.. a couple of times..