I have got Blue Iris running and am fairly new to it.

I set up a couple of my cameras to continuously record. One of the cameras only keeps the last 5 recordings whereas the other one is following the limits set in the general recording settings. Is there a reason why one camera is dropping recordings after a time limit? Both cameras are identical.

For a start are the settings on the record tap for each camera identical, in particular are they both set to record to the default folder location usually "New"
What are your setting for Storage set too for "New" and any other folders that the files may get moved to due to age etc.
What version are you currently running, there have a recollection that a few versions in the past that had issues with storage retention (Not sure of the particulars)!

Have you done a DB rebuild to see if this corrects the issue

To many possibilities as to what it might be but 10Gb for New is way too small especially so first thing I would do is try increasing that.

Also New on C: where your Os is, not something that I would do.

If all you are doing is moving files from C to D then don’t bother moving things around, there really is no point so configure all the space in either New or Stored on D and then write the clips directly there and let BI then manage the space but not based upon age of clip.