Are you sure that you had Seagate EC drives, and not Seagate Archive drives? The latter are DM-SMR and therefore poorly suited to pretty much anything doing random write I/O (e.g. rsyncing directory trees, rdiff-backup and so on). However, with the correct workload (object store, log-oriented storage) they perform admirably, especially for their price.

With DM-SMR strange noises and paranormal drive activity has to be expected since the drive can spend considerable time flushing it's PMR buffer (~16 GiB) to the shingled zones.