Slow Differential Backup of Raid-5 Archive

I have a Raid-5 2TB array with just data on it. I have Paragon HDM 12 Pro. The Raid sector based archive is 731 GB. I am doing the differential backup to a 1TB USB 3.0 portable drive via USB 3.0. The sector based archive is on a similar portable drive but connected via USB 2.0. The original sector based backup took approximately 10 hours to complete. It was on a USB 3.0 connection. So far, as of the time of this post, the differential backup of the sector based archive has been running for 9 hours and the speed indicator says the differential backup will take more than 12 more hours to complete. Even more of a puzzle is the size of the differential backup. Right now it is 320 GB and growing. This after the initial sector based backup was completed only 4 days ago and I know I have not touched/added 320 GB of data. Paragon support so far has not been able to provide any answers.

I don't know if the size of the data to be backed up/differential backed up has anything to do with the speed issue. Also, the fact that the original sector based archive is on a USB 2.0 connection and the differential backup is to a drive on a USB 3.0 connection if that has anything to do with this speed issue.

Any thoughts out there?

Thanks,

Phil