How to recover the name of a non-system volume containing all user folders in Mac OS X Lion?
The fastest way to solve your problem is probably adjusting the path of your user directory in the yourusername.plist (assuming following current path of your user folder: "/VolumeNameContainingYourUserFolder/Users/yourusername"):


Boot to Recovery Mode.
Open Terminal.app in the menubar (Utilities -> Terminal)
Enter df to get the paths/names of your main system volume and the volume containing your user folder. You may have to move around with cd and check the content of folders with ls if you don't remember the full path of your user folder.
Change your working directory to the folder "users" deep inside /private:

cd /Volumes/NameOfYourMainSystemVolume/private/var/db/dslocal/nodes/Default/users/

List the content of the folder:

ls

Locate your username.plist and convert it to xml:

plutil -convert xml1 yourusername.plist

Then modify the file with vim* (please consult online resources how to work with vim first!)

vim yourusername.plist



Modify the string 

/Users/yourusername  


or

/Volumes/WrongVolumeNameContainingYourUserFolder/Users/yourusername


to

/Volumes/VolumeNameContainingYourUserFolder/Users/yourusername


in the key "home" at line ~90.  
Save the changes and quit vim.

Convert the plist to the original binary format:

plutil -convert binary1 yourusername.plist

enter exit and quit Terminal
Reboot to your main volume, log-in with your account.


Optional: Enable the root user temporarily. Log-out of your user account and log-in with root.
Make any necessary adjustments like renaming the volume containing your user folder (or other user folders) again and changing the path(s) to the user folder(s) of your main user (other users) in System Preferences accordingly.
Log-out and log-in with your common user again. Finally disable root.



*: It's much easier to use nano or pico instead of vim, which has two different modes: command mode and insert mode. While testing the solution in a Recovery HD of a virtual machine, i first tried nano yourusername.plist, then  pico yourusername.plist. Both commands gave me an error: Can't find nano/pico. So I decided to use vim, because I was too lazy to find out if nano or pico are present in the Recovery HD at all. You may try nano instead: Use it like an almost common text editor by moving around with the arrow keys and adding or deleting text as usual. After finishing your edit, hit ctrlo to write the changes to disk and ctrlx to exit nano.