How to rename a partition in OS X?
To rename the volume, enter the following in Terminal:
/usr/sbin/diskutil rename oldname newname
for example:
/usr/sbin/diskutil rename "Macintosh HD" "Mac Disk"
You can also right-click a partition in Finder (in Computername at the top level of the hierarchy), select Get Info, and enter the new name there.
Or just go to Finder and change the name in the left hand column, just like a file.