The previous replies addressed only the secondary issue of securing your hard drive not the fundamental issue of the inability of C1pro to save to two locations when tethered.
I use a laptop (PC) when tethered and have a 500Gb SSD plugged into a USB port as the secondary storage. A while ago a "nerd" wrote a small text batch file that copies files from a designated folder to another which is on my SSD. All this is similar to what DropSync is doing for you.
I like the SSD option rather than an SD card simply for it's storage size, these days 32Gb is not that much. The SSD, while small, is still a lot bigger than a camera card, faster to download and harder to misplace. Mine is a nice bright blue, too.
The problem is not whether someone velcros a drive to a table or not but that additional functionality is needed in C1pro when using a tethered setup.