iPhone/iPad back up to different hard drive?
Sure, if you are on a PC, create a junction. 


Locate your current iTunes backup folder. If you are using Windows 8, 7, or Vista, to locate the iTunes backup folder use the Windows Start Menu and enter the following in the Search box: %APPDATA%\Apple Computer\MobileSync
Move the current iTunes iOS backup folder to the new desired destination via a simple drag and drop 
At the original folder location, press the SHIFT key, right-click and select “Open command window here” 
In the command prompt window, manually type in the following command where E:\ is the new drive location (drive letter may vary) and “Backup” is the name of the iOS backup folder you just moved: mklink /J “%APPDATA%\Apple Computer\MobileSync\Backup” “E:\Backup”
You will now see the command prompt window return a success in creating a junction 
A new folder shortcut called “Backup” will appear in the original location 
Double click on the “Backup” folder shortcut and while it may seem you are still within the MobileSync folder on the C: drive, what you actually see is the contents of the “Backup” folder you previously moved to the new location. You can think of it as a virtual tunnel created through your computer directory space