How to find bad drive in array

Solution
If you interrupt the BIOS sequence at the point at which you configure the RAID setup, the RAID manager may show you the failed drive. Depends - what RAID controller and / or motherboard are you using?

And you do have backups, right?
You'd have to run a hardware scan on each drive separately. What to you mean by "boot scan"? This a drive scan utility in the BIOS or something the RAID firmware does? The drive vendors should have utilities to scan the drives.

4 drives in RAID 0, you must like to live dangerously with your data. Hope you keep good backups.