When testing hard disk with SMART (for instance using smartctl tool) I can do tests (for instance long test) in two modes: captive (also called foreground mode) or non captive. Non captive mode is more convenient, because it does not block the hard drive. So what is the purpose of captive mode? In what circumstances should I prefer it over non-captive mode?
Captive mode tests run faster as the HD is doing nothing else... When testing is the first priority you should run that. If the HD is otherwise "in use" you should run the non-captive mode testing, so that your Server doesn't see the HD as dropped or something.