Backup to USB fails due to lack of space; but it's an empty drive and there are no files on it!

I've upgraded my NV+ v1 (4.1.14) with 4x 2TB drives and I ran into the 4k block size problem. So I'm trying to backup my device so I can factory reset it.

The current size of the volume is 1.3TB, so I took out one of the 2TB drives to use as a backup via USB. 1.3TB should fit easily into 2TB, right?

I added the 2TB drive to the front USB port and formatted it as ext3, it shows up just fine in the shares and has the expected free space.

So I made a backup task and source=share/ dest=USB/Sharename/ and ran it.

it runs for a few minues and stops with this error: 

Error encountered copying data from source path ==> /USB_HDD_4_1//blah due to lack of disk space. [Job 003]

I look at the USB drive and it is FULL. Zero bytes available. It has made a few folders, but they are all empty.

What is going on?

Are there hidden files (.appledouble, etc)? I ran BlueHarvest on the volume before the backup and it deleted a bunch of these. But not 2TB's worth.

I use this volume mainly with my mac... is there anything I'm missing? 

 Any help is appreciated! Thanks