the hard disk on the computer contains your archive of technics and midi files (and other files that will load into the 7000).
the 7000 loads from SD card or floppy.
so you transfer your files to SD card first, then have them available for loading quickly in the 7000.
there is no reason to have all your archive on a big SD card unless you want absolutely everything instantly available. Even on the SX you waited a few seconds to change partitions - no different to flicking a card out and slotting another in!
you can copy new stuff everyday on to a small card.
or have several small cards, one in the 7000, one in the pc always ready for copying the next stuff you want to play.
find the best way of working for you before spending lots of money on cards.
as long as your archive is on the pc hard disk, and you back it up to cd or whatever else you have available, it is safe, and you can get it to a card and into the 7000 pretty easily.
Let's face it the cards are so small you have to be careful not to lose them. Maybe get a little wallet to keep them in!