> 
> Why, when I try to make a directory, do I get the following message:
> 
> mkdir testdir: Read-only filesystem
> 
> Can someone help me?

Please see:

http://www.macbsd.com/macbsd/macbsd-docs/faq/faq-5.html#ss5.4

When ever you boot in single user, the root fs is read-only, so that
you can fsck it.

Take care,

Bill