My dashboard says one of my drives has failed, and need to replace. Now I'm fine replacing it, but I don't know which ones I need to replace. Without testing one by one by yanking those drives out of the bay, are there anyway for me to know exactly which failed?

The Main page shows the serial number for each drive. The serial number is printed on the drive label.

Depending on what kind of case you have, many people user a label writer and print either the last few digits of the serial number or the entire serial number for each drive and stick them on the drives so they are easily visible when you open your case. This obviously helps locate drives when they fail and need to be replaced.

If you have activity lights for each drive one ‘trick’ that can work is to start a parity check.   The one that does not come on is the failed drive.   Once the drive is identified you will want to cancel the parity check.