Seems to me that if you have another drive with lots of free space that is the most obvious destination for your files.
you certainly don't need a new drive ... and it doesn't matter if you are using usb or memory cards - what matters is where you put the files
if you move my documents using the method I gave
http://support.microsoft.com/kb/310147
windows changes the %my documents% variable to reflect the changes and so any and all applications will "see" the destination (I've used this method without problem since win2000)
also by moving the entire "my docs" folder you will free valuable space on your system drive
(ntfs and windowslike about 20-25% free space to operate efficiently)