I added a custom toolbox created for 9.3.1 to a ArcGIS Desktop 10 ArcMap. Now it seemed to have disappeared. I can't add it to ArcMap 9.3.1.
Is there a way to convert it back if that is even the issue? Or is there something else going on?
Right-click on the toolbox in ArcToolbox and select Save As specifying 9.3 as the outgoing version. I ran into this problem previously, it is wise to copy your 9.2, 9.3 etc toolboxes to a new location prior to loading into version 10.
I should add, that user toolboxes won't be available by default in version 10 even though they may have been so in 9.x. You will need to re-add them to make them available.