This really doesn't have anything to do with your question, but I might never get another opportunity to point this out. There is a weapon in the game called "Prophets of Motav", a wand weapon that is a tome with some symbol on the cover. The weapon's flavor text reads:
"An ancient document left by the developers of techniques.
There may be secrets hidden inside..."
I only have my speculation as a fan, and not any confirmed lore, since all the Phantasy Star games love to hint at each other while the developers state they take place in a separate universes with similar themes. Techniques were first introduced in the series in Phantasy Star 2, which differentiated from Phantasy Star 1 where magic was used. The use of magic was 'lost' over the thousands of years between the original games until a character was able to use it in Phantasy Star 4, and in the meantime humanity developed Techniques.
In PS2, the game primarily takes place on a planet called 'Motavia', sometimes just called 'Mota' due to limitations. I don't think it's a stretch to assume that techniques were created on Motavia, and the Prophets of Motav was written there. The names are similar enough!
As for why disks... Well, I can think of game balance reasons they went with it as opposed to learning stronger techs on leveling up, but PSO's always been a fantasy world with a sci-fi coat of paint. Tomes and scrolls are oldschool. In a world of space travel, teleportation, and laser swords, you gotta have information stored on disks!