Overview Generally with the Nikon film scanner and the SA-21 film adaptor you can only scan 2 to 6 negative strips. The SA-30 is a bulk film holder that can handle strips of up to 40 negatives. The SA-21 can be "modified" to flake a SA-30.
This is taken from a blog: https://groups.google.com/forum/#!topic/comp.periphs.scanners/-LdJf3r23h4 by Kennedy McEwan Apr 25, 2005:
Incidentally, for those who do not want to take the risk of opening their SA-21 up and making a permanent solder link modification, there is an alternative temporary fix, albeit a little less reliable.
The two contacts of the link in question connect directly to pins 16 and 17 of the blue rear connector on the SA-21. Looking into the rear of the SA-21 with the adapter the correct way up, pin 13 is on the middle row on the left of the connector, so just count 4 and 5 pins across and you get to the two pins in question. These can be shorted together using some very fine wire (I/John used a strand of copper wire from speaker wire ....length of about 14 mm) - thin enough so that the operation of the connector contact is not interfered with. Then hold the link wire in place with some adhesive tape - Scotch Magic tape works quite well for this.
Before pushing the SA-21 back into the scanner, make sure that you pierce the adhesive tape over each contact in the connector block with a pin. This will prevent the corresponding pins in the scanner being subjected to additional pressure as the adapter is pushed home in the scanner.
I took a black sharpie and highlighted the area around pins 16 and 17.