If you have a computer with the minimum 32 megabytes of RAM and also have a large RAM disk defined, say 75 percent of available memory, then Mac OS 9 will freeze or behave strangely upon restarting after upgrading.
The computer will start up and then either freeze at the Happy Mac or display a grey screen and freeze. The dedicated RAM disk does not leave enough memory for Mac OS 9 to run. It's possible to recover from the startup freeze by restarting and resetting PRAM.
This issue can be avoided by reducing the size of the RAM disk before installing Mac OS 9. Larger memory configurations should not encounter this situation.