Question about LC-Power LC-35U3-RAID-2

Good day, I have the following question about the LC-Power LC-35U3-RAID-2 in RAID 1 configuration: If one of the two hard drives fails and I replace it with a new (empty) hard drive: Does the rebuild start automatically or do I have to perform manual steps (e.g. press reset button)? Which LED indicators signal that a rebuild is in progress? Can I use the device during the rebuild or should I not use it? Unfortunately, the manual does not cover this scenario in detail. Thank you very much for your help!