Why is a subst drive usable from a command prompt but not Windows 7 explorer

I have a registry key under [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run] The value of the key is

"c:\windows\system32\cmd.exe" /c subst t: e:\MyFolder

More often than not when I boot the computer the subst command appears to only half work. If I open a command prompt I can use t: fine. However if I look in windows explorer T: is not there and if I type it into the address bar I get an error message saying "Windows can't find 't:'. Check the spelling and try again."

This is on Windows 7 Enterprise 64 bit.

Any ideas?

This command creates a persistent Z: subst of C:\DirectoryName

reg add "HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices" /v Z: /t REG_SZ /d "\??\C:\DirectoryName" /f

This probably happens because the run command runs elevated (as administrator). If you run the subst command not elevated you will see it in Windows Explorer. You can test this running the subst on an elevated command prompt.