If the drives are accessible from the outside/front of the machine, then they are hot-swappable, and if they are hot-swappable, drives should ALWAYS be replaced "hot" (while the server is running). You can run in degraded mode until another disk fails, at which time, your array and data will be lost, so replacing a failed drive should be done as soon as possible.
Hello, we have DELL R710 Server and it's showing as below, i can see the all data on the server (configured as RAID5), i can still work on the server as before  
Need Help in DELL R710 RAID DISK Failed