Any way to change the location of Time Machine local backups (./MobileBackups) to another drive?
I couldn't find any way of modifying the path /.MobileBackups, but you can create a folder on your HDD and then symlink it to /.MobileBackups:


Open Applications>Utilities>Terminal. 

Now you will have to type several commands. Press return after every command for it to execute. When asked, type your password.
The snapshots folder has complex permissions. The simplest way to preserve them is to copy the existing folder to the new location on your HDD:

sudo rsync -ahvHE /.MobileBackups /Volumes/HDD/

(replace /Volumes/HDD with the path to your HDD.) 

rsync options are:


-a: archive mode (recursive; copy symlinks as symlinks; preserve permissions, times, owner, group, device files, sockets and fifos)
-h: output result in human-readable format
-v: verbose, that is, show what the command is doing 
-H: preserve hardlinks
-E: copy extended attributes like resource forks and ACLs

Disable local snapshots (this will delete /.MobileBackups and its contents):

sudo tmutil disablelocal
Create symlink:

sudo ln -s /Volumes/HDD/.MobileBackups /.MobileBackups

(replace /Volumes/HDD with the path to your HDD.)
Enable local snapshots:

sudo tmutil enablelocal
Create a local snapshot:

sudo tmutil snapshot
If you see a new snapshot dated "now":

ls -l /Volumes/HDD/.MobileBackups/Computer/
(...)
drwxr-xr-x@ 3 root  wheel  102 Jan 28 09:05 2013-01-28-090534


you're done!


If something went wrong or is not working as expected, revert to the original configuration:


Disable local snapshots:

sudo tmutil disablelocal

This will delete the symlink /.MobileBackups. Should that not be the case, delete it with:

sudo rm /.MobileBackups
Enable local snapshots:

sudo tmutil enablelocal
Delete alternative snapshot folder on your HDD:

sudo rm -rf /Volumes/HDD/.MobileBackups

(rm -rf is a dangerous command that won't ask for confirmation before deleting the folder. Make sure the path is correct!)