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Personal.xlsb

Personal.xlsb is a special Excel Workbook whose macros are available to all opened Excel workbooks.

Excel itself refers to Personal.xlsb as the Personal macro workbook when asked if modification to it should be changed.

Default location

The default location (directory) for Personal.xlsb seems to be %appdata%\Microsoft\Excel\XLSTART. This directory can be determined in VBA with application.startupPath.

When such a Personal.xlsb has been created, it will automatically be opened (and visible) whenever Excel is started.

Personal.xlsb can be made invisible when Excel is started by using the View -> Window -> Hide menu.

In order to make changes to Personal.xlsb, it should be made visible again.

With VBA, Personal.xlsb can be hidden like so:

workbooks("Personal.xlsb").windows(1).visible = false

Apparently, a Personal.xlsb workbook can become disabled. If so, it can be enabled again with

File -> Options -> Add-Ins
At the bottom of the dialog is a drop down, named Manage, from which Disabled Items must be chosen
Press Go
Enable Personal.xlsb if it is in the list.