Logical mistake in the chapter 2.6 Allocating Disk Space

[Oleg Fedjakin]

Hello!
Lately I consult newbie about slices and partitions in FreeBSD. In the
"An Illustration from the Files of Bill and Fred's Exceptional
Adventures" we read that "When Bill re-ordered the SCSI BIOS so that
he could boot from SCSI unit four, he was only fooling himself.
FreeBSD was still running on SCSI unit zero". But early we read:
"older (zero) SCSI drive is reporting numerous soft errors".

Question: if FreeBSD boot from errored SCSI and works with it, why OS
works safely? This drive is unstable. Therefore nothing change. Or
not?

Thanks for answer!

Best Regards from Russia!

FreeBSD-user Oleg A. Fedjakin