Based on feedback that I am now getting, there is a good possibility that some Macs updated to Mac OS 7.6.1 have a problem where selecting an internal drive to be the startup disk (as selected in the Startup Disk control panel) does not "stick" - and that this may account for the half-minute startup delay (as reported previously). More specifically, Darren Burgos noted that, after selecting his internal drive as the startup disk, it remained properly selected for subsequent warm restarts (as selected via Restart from the Finder's Special menu) but not after a complete shut down and cold start from the Power key. In such cases, as reported by other readers, Apple System Profiler continues to list the startup disk as "unknown."
Update: This problem appears not to affect most PCI-based Macs. It appears most likely to occur with Macs with internal IDE drives. The "unknown" listing may occur even if the Startup Disk control panel setting is correct; this may be due to a problem with Profiler itself.
Still others have claimed that their internal drive is properly selected as the startup disk but the "half minute startup delay" remains. One reader (Peter Spomer) claimed that his blinking question mark startup problem (which only happens on warm restarts, as described previously) was similarly not affected by the Startup Disk selection.
And, of course, one reader confirmed that selecting a startup drive from the control panel did work to fix their half-minute startup delay.
Danny Jewell said that a PRAM zap fixed his delay (don't forget to reset the Disk Cache size after doing zapping!).
This is getting too confusing for me. It is as if there are three or more different causes for the same odd symptom.
And all of this remains separate (as far as I can tell) from the virtual memory related delay, also reported previously.