When you copy a text document to a LPT port (COPY <Drive:>\Folder\FileName.TXT LPT1:), you may receive an error:
All pipe instances are busy.
If you close the CMD prompt and re-open one, the COPY generates the same error.
This problem will occur if the COPY command was issued at the same time that the spooler starts to print a spooled print job.
To fix the problem:
net stop spooler
net start spooler