updated to simplified C structs.

writing struct to file

#include <FS.h>


void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println();
  Serial.println();

  struct Person{
    char personId[10]="1234abc";
    char firstName[10]="Steve";
    char lastName[10]="Nelson";
  };
  
  Person me;
  Serial.println(me.personId);
  Serial.println(me.firstName);
  Serial.println(me.lastName);

  SPIFFS.begin();
  File writefile = SPIFFS.open("test","w");
  writefile.write((uint8_t *)&me, sizeof(me));
  writefile.close();

  Serial.println("file written!");

  Person oldMe;
  File readfile = SPIFFS.open("test","w");
  readfile.read((uint8_t *)&oldMe, readfile.size());
  readfile.close();

  Serial.println("file read");
  
  Serial.println(oldMe.personId);
  Serial.println(oldMe.firstName);
  Serial.println(oldMe.lastName);
  
  
}

void loop() {
  // put your main code here, to run repeatedly:

}

This now works when I remove the String datatypes. What I've learned is that this technique only works with simple data structures.