very slow on large external drive

I am reducing the size of a partition on a 3TB external drive - if first reported it would take 5 hours but this has continued to rise and it's now showing it will take another 18 hours to complete.

At first I thought it had stuck at 40% but left it a few hours and it has climbed to 47% so is working but oh so slowly?

Is this normal, what worries me is that I am creating space to temporarily create a new encrypted drive and I will then want to copy that encrypted drive partition back to another internal drive - at this rate it could take me weeks

Anyone have any ideas why this should be so slow - I have no indication of what normally to expect

I am using Partition Manager 11 free -
The drive is an external 3tb seagate go flex which before I started had one large partition with about 950GB of free space I wanted to reduce the main partition to around 2500 GB so that I could temporarily create a new partition on this - then copy the data from its existing location, add some additional data and then delete the original partition and copy the new partition back to the old drive.

If I had known it might take this long I would have just deleted the partition on the external drive (which is just my back up) and re-created two new partitions - I had just thought this would be quicker than formatting the drive again. I could try to kill the process but I am worried in case I then brick the drive for good, and am not able to reformat it as while this process is running I cant see the drive in windows

Anyone have any ideas why this should be so slow - I have no indication of what normally to expect, and even if it succeeds how long is it going to take to copy the newly created partition back to the main drive and then restore this partition to its full size?