Save an array with custom classes

Okay, I've looked around but can't find anything that helps me out with this issue:

I have a list, containing several custom classes, identical in layout to the example below which is a snippet from one single file, OldSystem.cs :

 using UnityEngine;
 using UnityEngine.UI;
 using System;
 using System.Collections;
 using System.Collections.Generic;

 [Serializable]
 public enum DriveType {
     Floppy,
     HDD,
     SSD,
     Optical
 }
 
 [Serializable]
 public class File {
     public string fileName;
     public int fileSize;
     public List<string> contents = new List<string>();
 
     public File(string fName, int fSize)
     {
         fileName = fName;
         fileSize = fSize;
     }
 }
 
 [Serializable]
 public class Drive {
     public string letter;
     [SerializeField]
     public DriveType driveType;
     [SerializeField]
     public List<File> files = new List<File>();
 
     public Drive(string assignedLetter, DriveType type)
     {
         letter = assignedLetter;
         driveType = type;
     }
 }
 
 [RequireComponent(typeof(AudioSource))]
 public class OldSystem : MonoBehaviour {

     [SerializeField]
     public Drive[] drives;

     // Rest of main script code here
     ....
 
 }

I would like to be able to save the drives list from the OldSystem.cs MonoBehaviour somehow, as it is modified at runtime (either in the Editor or in the standalone build - but mostly the Editor at the moment) so that when I go back to the Editor, the changes are persistant.

As you can see, I've used a scattergun loaded with [Serializable] statements on the script, but to be honest I don't really understand what they do or how serialization works.

I would be more than happy if someone can point me in the direction of a script or tutorial or some guidance on how to save the drives list to an XML file or other type of file in the Assets folder, perhaps - so long as that approach would work with a standalone build as well.

Otherwise, any help or suggestions would be gratefully received. I'm certainly not a beginner at programming, but I'm very inexperienced at this particular sort of thing (serialization, getting Unity to save data etc.)

Have I even declared the DriveType enum, File class or Drive class in the right places?

Thanks! :D