Thanks for the prompt replies Lennart.
The source disk read issue is not limited to Retrospect. It also happens for two other backup/duplication products that I'm trailing, namely Time Machine and Memeo.
I've tried HFS+ and NTFS (via Paragon driver) as targets, but both have the same issue. So it must be source disk/file-system or O/S related. I've tried two different internal SSD source disks - same problem.
Anyone have any ideas why MacOSX would behave this way?
Does anyone else see this?
If not - then I assume something else that I have installed must be causing it - but I'm new to MacOSX and I don't know where to look.
I've never seen anything like this before on any O/S (OpenVMS, Solaris, linux, WinAll), or backup/duplication/cloning software, i.e. where software is able to read source disk fine for 2 to 10 minutes and stream to the target disk, and then nothing, it all goes quiet for 2 to 10 minutes. Most odd. The read/write duration and the delay duration both vary randomly from anything from 2 minutes to 10 minutes.
If I run any of several disk exercisers - these run fine on both source and target - but then these work on just one file. It's almost as if the file system has a problem processing a large amount of file reads - or OSX is detecting a large amount of file system reads and stepping in and pausing.