You may not be able to determine this with certainty until you troubleshoot.Troubleshoot software before hardware unless the issue prevents any software troubleshooting.That's the dialog box that appeared when I rebooted (it included an explanation that began "Caches necessary for booting are being updated ....").I normally don't reboot (almost always simply put the i Mac to sleep, rather than shut down).-- i Mac (24", 2.8 GHz Intel Core 2 Duo, 2GB RAM, 320 GB HDD) OS X (10.5.4) The system caches information about what kernel extensions need to be loaded at startup, and actually even caches binary information from those extensions so it's all in once place.This speeds up the boot process, since the system doesn't have to figure out what to load and then load it from hundreds of tiny little files.
I'm not at all convinced a modal dialog was the best way to do that... As to why Apple decided to show the message, the only explanation I have is they thought it was important that users know why the restart/shutdown was slow.
Inevitably, when there's a new release of an operating system there are going to be problems, but don't let the list below put you off, as most people should find the process of upgrading to mac OS Sierra pretty painless.
If you haven't upgraded already, check out our guide How to download mac OS Sierra.
But after installing a driver for an old mouse, I needed to reboot, and received that dialog Why would I get such a message?
I've never had to "update" any kind of "cache" before, nor am I aware of the system doing it for me.