I don't think this will work. Besides matching the connection (supposing you could use the PCMCIA to USB adaptrer card), you also need the computer to recognize it (have the right drivers), which it does not, unlike the PC, where a similar setup would work fine.
However, why would you want to use a hard drive? Flash cards are much more reliable, not having any moving parts, and are contained within the instrument (rather than sitting separately outside the keyboard). Besides, a hard drive may be useful to store audio/MP3 files, or sound samples, but the G70 can make use of neither of those. Any flash card (you can buy one up to 4GB in size) will store plenty of MIDI files and styles.