mount 2nd HD containing running Freebsd system

[Olivier Nicole]

>>> I have run out of space on my running 7.1 system. I have a 2nd HD 
>>> containing 6.1 that I want to mount and use the 80gb /usr slice.
>>>
>>> What is the mount command to use?

In order to give you the right answer, we would need to know how that
disk was organised:

- slice (of fdisk type); use the command fdisk -s /dev/da1

- partitions (or disklabel); use the command disklabel /dev/da1s1 (if
  the slice with FreeBSD is the slice 1)

- Now you may have used your disk witout slice, I cannot help, or
  without partition (like Linux), I cannot help either.

You can also run /sur/sbin/sysinstall. In Configure choice, you will
have two menus, Fdisk and Label, that does the same thing as the
command fdisk and disklabel.

Only when you know what you want to mount, we can tell you how to
mount it.

Best regards,

olviier