My everyday laptop HDD is now holding 900 GB.  I have  been backing it up by (1) cloning to a duplicate drive &/or (2) creating an image on a 3.5" HDD which is kept elsewhere.
   Either process works well, except for one thing - either process takes over 2 hours.   :-/  Simply copying a 900 GB HDD image from one HDD to another is taking over an hour, and this is only at the OS level, not involving the actual cloning/imaging software. 
   I suspect the limitation is the bit transfer rate between the HDD's and not in the cloning or imaging software.   A bulk data transfer seems to be about 50% of the problem.
   I would be willing to invest a bit to buy hardware that can do this job in less than an hour, such as dedicating a motherboard/CPU with at least 2 SATA3 ports linking the HDDs to speed up the transfer (if doing that would help).  I have a couple of old desktops, but they are limited to SATA2 ports.

@ artfd1

That sounds like a fast laptop. Backing up 900 GB in over 2 hours sounds excellent.

Which imaging app are you using? What size is the backup image?

Which OS is installed?

How many partitions are on the HD? What size are the partitions? What size is the HD?

How is the 3.5" HD connected to your laptop when you are creating an image?