performance when copying large files

This has been discussed quite a lot before, but it
continues to interest me. I create a full image backup of
my system once a week and an incremental backup each of
the next six days. Backups (Acronis) go on a spare HD
partition and I then copy them to an external (WD)
Passport 2GB drive. Copying the full backup image takes
over an hour. What I've noticed, usually, but not every
time (!?) is that the transfer gradually slows down. See:
https://imgur.com/hESFJq1

While the copy proceeds there is no other activity beyond
the usual Windows background stuff running - no apps,
browsers, etc.

What might account for this?