Some of the communication stuff I worked on in the military and later used this type of memory. I was once told that they were hand "sewn" by Asian women because they were the only people with the eyesight and steady hand to do this work. Vaguely I remember something about having read, write, and re-write functions. The beads required two lines to make them change state...X and Y. Either didn't provide enough current to switch them so this happened only at the intersection where both passed through a bead. To read from memory, an attempt was made to switch each bead back using the same method. If it switched, a small current was felt in the read line. This, effectively, cleared that bead and the controller would need to write it back again....or at least that's the explanation I remember. I'm thinking that 8 or 16 k occupied nearly 1 square foot of space on a PCB. I never saw one of these fail.