file.system <- list(
  `Disk 1` = list(
    `folder_3500` = list(
      file1 = 1,
      file2 = 2
    ),
    `folder_7901` = list(
      NULL
    ),
    `folder_5383` = list(
      file1 = 10,
      file2 = 11,
      file3 = 12,
      file4 = 13,
      file5 = 14,
      file6 = 15,
      file7 = 16,
      file8 = 17
    )
  ),
  `Disk 2` = list(
    `folder_1500` = list(
      file1 = 100,
      file2 = 101,
      file3 = 102,
      file4 = 103
    )
  ),
  `Disk 3` = list(
    `folder_5320` = list(
      file1 = 1000,
      file2 = 1001,
      file3 = 1002,
      file4 = 1003
    ),
    `folder_9666` = list(
      NULL
    ),
    `folder_1996` = list(
      file1 = 10,
      file3 = 12
    ),
    `folder_3509` = list(
      NULL
    ),
    `folder_8189` = list(
      file1 = 110,
      file2 = 121,
      file3 = 132
    ),
    `folder_6552`  = list(
      NULL
    )
  ),
  `Disk 4` = list(
    NULL
  )
)

for (i in seq_along(file.system)) {
  names.disk <- names(file.system)[i]
  cat("-", names.disk, "\n")

  folder <- file.system[[i]]
  for (j in seq_along(folder)) {
    names.folder <- names(folder)[j]
    cat("---", names.folder, "\n")

    file <- folder[[j]]
    for (k in seq_along(file)) {
      names.file <- names(file)[k]
      cat("-----", names.file, "\n")
    }
  }
}

## TODO this can be recursively done, it's handy especially
## if the depth of the tree (number of levels) is unknown.