Recording compressed video, particularly if it is not continuously recording, is actually a fairly light load for a hard drive. Consider a heavy camera load of 100 Mbps is actually only 1/10th (give or take) of a modern hard drive's write speed. All that fragmentation really does is slow down read speed, and to some extent write speed (if the free space is also fragmented). It would take some truly horrific fragmentation to make the drive unable to cope with a typical
Blue Iris load. I wouldn't worry about defragmenting the HDD.
There could be some benefit to saving clips on the SSD first, since the files must grow continuously during recording, this opens the door for fragmentation. By the time they move to the HDD, the final file size will be known. As a bonus, viewing clips off the SSD should be
a little bit snappier than off the HDD. But if it is a lot of clips, you could just end up wearing out the SSD prematurely.
