The README.TXT that comes with Microsoft Windows version 3.0 includes
Omit the APPEND, JOIN, and SUBST MS-DOS Utilities
The MS-DOS utilities APPEND, JOIN, and SUBST can cause problems
when you are running Windows 3.0. If you included APPEND, JOIN,
or SUBST in your AUTOEXEC.BAT file, remove them and refrain from
running them within Windows.
If any of these commands are in the AUTOEXEC.BAT file when Microsoft
Windows 3.0 is installed, they can cause the installation to be
corrupted. This can cause the unrecoverable application errors (UAEs)
to continue to occur even after the APPEND, JOIN, and SUBST commands
are removed from the AUTOEXEC.BAT file.
These commands can also cause intermittent UAEs if they are added
after the installation of Windows. Therefore, customers who use
Microsoft Windows 3.0 should be warned when these commands are added
to the AUTOEXEC.BAT files.
The MS-DOS utilities APPEND, JOIN, and SUBST are listed in the
[incompTSR2] section of the SETUP.INF file. If you are running these
programs when you run Windows Setup, you receive the following
SETUP has found the programs listed below on your system. When
SETUP or Windows runs with some versions of these programs, your
system may fail. It is recommended that you quit Setup now and
look in the file SETUP.TXT for information about using the programs
with Setup or Windows. Then run Setup again.
For more information on the SUBST, JOIN, and APPEND commands, query on
the following words:
subst and path and msdos
subst and join and append