Is your recording software or hardware based? Hardware based tends to work even with the system being used for other things. sometimes software needs more free system resources since the cpu is doing all the work.
A dedicated drive has to be just a drive, not a partition on a drive(if nothing else is using the drive, it should be ok to use the existing partition).
If the drive is external, you may also run into some problems of bandwidth limits(depending on the video bit rate.).