It all depends on the controller/connector.
I am assuming USB 2.0. That is slow and if both device hand on the same controller the speeds is cut in half because reading and writing takes bandwidth.
3 days is a lot.
Are you using NTFS-3G by any chance and writing to NTFS volumes. NTFS-3G is free but horribly slow. Either change partitions to exFAT or buy a decent NTFS driver like Paragon's.