Diagnosing FileVault problems
My startup disk is encrypted using FileVault, which worked fine, but I’m now having problems getting into it when my Mac starts up. I fear that I’ll need to decrypt it and start again. Is there any way to diagnose the problem from the Mac’s log? You might be lucky, but when using FileVault you may well be unable to read any log entries from before the disk is unlocked. Normally, from Sierra onwards, the beginning of the startup process is marked by a log entry containing the message system boot:, while earlier systems use SYSTEM_BOOT instead. Locate the time of that, then view log entries from perhaps 10 seconds before that for the next minute, looking for those that appear relevant.
The problem is that before the startup disk is unlocked, your Mac can’t write any entries to the log. What it might do is record these in memory, as it normally does when keeping the log, and once the disk is unlocked it might write those contents to its log files on disk. If that’s the case, this is probably the only way to discover what’s going wrong. Otherwise you’ll almost certainly have to disable encryption and see whether the problem then goes away.