Publisher Summary This chapter discusses the problems that are at the high end of both scales—system failures, which take an entire system out of commission until corrected. These problems can be divided into three principal classes—boot failures, system crashes, and system hangs. The troubleshooting tools that come with Tru64 UNIX require Tru64 UNIX to be up and running. The troubleshooting capabilities available at the console firmware level are much more limited in scope. If a system crashes during the boot sequence, there are five general types of errors—boot disk access failure, inability to open osf_boot, kernel load failure, machine checks, and panics during boot. These are distinguished by the type of error message displayed before the system returns to the console prompt. When a system hangs while booting, it is necessary to determine what it is doing at the time of the hang. One should observe the last message that appears before the system hangs. This message could be the operation causing the hang, or it could be the last successful operation to complete before the hang.