Lua - FlashAir card CONFIG file gets damaged

Question

I am using Toshiba FlashAir card to wirelessly upload photos from my drone to my web server via Lua script installed on FlashAir card. The card works fine most of the time, but sometimes the photos doesn't upload. When I connect to the card via my mac, I get this message, "CONFIG" is damaged and can't be opened.

Not sure why this is happening. The ftpUpload.lua script that does the uploading is listed below:

require("Settings")

local blackList = {}
local dateTable = {}
local parseThreads = 0
local ftpstring = "ftp://"..user..":"..passwd.."@"..server.."/"..serverDir.."/"
local lockdir = "/uploaded/"   -- trailing slash required, and folder must already exist

local fileTable = {}

--Options
local sortByFileName = false -- If true sorts alphabetically by file else sorts by modified date
local jpegsOnly = true --Excludes .RAW files if set to true


local function exists(path)
    if lfs.attributes(path) then
        return true
    else
        return false
    end
end

local function is_uploaded(path)
    local hash = fa.hash("md5", path, "")
    return exists(lockdir .. hash)
end

local function set_uploaded(path)
    local hash = fa.hash("md5", path, "")
    local file = io.open(lockdir .. hash, "w")
    file:close()
end

local function delete(path)
    -- Both of the following methods cause the next photo to be lost / not stored.
    fa.remove(path)
    -- fa.request("http://127.0.0.1/upload.cgi?DEL="..path)
end

--Format file date to something meanigful
local function FatDateTimeToTable(datetime_fat)

    local function getbits(x, from, to)
        local mask = bit32.lshift(1, to - from + 1) - 1
        local shifted = bit32.rshift(x, from)
        return bit32.band(shifted, mask)
    end

    local fatdate = bit32.rshift(datetime_fat, 16)
    local day = getbits(fatdate, 0, 4)
    local month = getbits(fatdate, 5, 8)
    local year = getbits(fatdate, 9, 15) + 1980

    local fattime = getbits(datetime_fat, 0, 15)
    local sec = getbits(fattime, 0, 4) * 2
    local min = getbits(fattime, 5, 10)
    local hour = getbits(fattime, 11, 15)

    return {
        year = string.format('%02d', year),
        month = string.format('%02d', month),
        day = string.format('%02d', day),
        hour = string.format('%02d', hour), 
        min = string.format('%02d', min),
        sec = string.format('%02d', sec),
    }
end

--Looks for value in table
local function exists_in_table(tbl, val)
    for i = 1, #tbl do
        if (tbl[i] == val) then
            return true
        end
    end
    return false
end

local function create_thumbs()
    print("Creating Thumbs!")
    for i = 1, #dateTable do
        local message = "d="..dateTable[i]
        local headers = { 
            ["Content-Length"] = string.len(message), 
            ["Content-Type"] = "application/x-www-form-urlencoded"
        }
        local b, c, h = fa.request{
            url = "http://"..server.."/"..serverDir.."/ct.php",
            method = "POST",
            headers = headers,
            body = message
        }       
    end
    print("COMPLETE!")
end

local function upload_file(folder, file, subfolder)
    local path = folder .. "/" .. file
    -- Open the log file
    local outfile = io.open(logfile, "a")
    outfile:write(file .. " ... ")
    local url = ftpstring..subfolder.."/"..file
    local response = fa.ftp("put", url, path)
    --print("Uploading", url)

    --Check to see if it worked, and log the result!
    if response ~= nil then
        print("Success!")
        outfile:write(" Success!\n")
        set_uploaded(path)
        if delete_after_upload == true then
            print("Deleting " .. file)
            outfile:write("Deleting " .. file .. "\n")
            sleep(1000)
            delete(path)
            sleep(1000)
        end
    else
        print(" Fail ")
        outfile:write(" Fail\n")
    end

    --Close our log file
    outfile:close()
end

local function sort_upload_table()
    if (sortByFileName) then
        print("Sorting filenames alphabetically")
        table.sort(fileTable, function(a,b)
            return a.file>b.file 
        end)
    else
        print("Sorting filenames by modded date")
        table.sort(fileTable, function(a,b)
            return a.sortDate>b.sortDate
        end)
    end
end

local function run_upload()
    print("Starting upload")
    for i = 1, #fileTable do
        local ft = fileTable[i]
        print("Uploading:", ft.folder, ft.file, ft.dateString)
        upload_file(ft.folder, ft.file, ft.dateString)          
    end
    create_thumbs()
end

local function walk_directory(folder)

    parseThreads = parseThreads+1

    for file in lfs.dir(folder) do

        local path = folder .. "/" .. file
        local skip = string.sub( file, 1, 2 ) == "._"
        local attr = lfs.attributes(path)
        local dt={}

        if (not skip) then
            print( "Found "..attr.mode..": " .. path )
            if attr.mode == "file" then
                local dateString = ""
                if (attr.modification~=nil) then
                    dt = FatDateTimeToTable(attr.modification)
                    dateString = dt.day.."-"..dt.month.."-"..dt.year
                end

                if not is_uploaded(path) then
                    if (not exists_in_table(blackList, dateString)) then
                        local s,f = true, true
                        if (jpegsOnly) then
                            s,f = string.find(string.lower(file), ".jpg")
                        end
                        if (s and f) then
                            fileTable[#fileTable+1] = 
                            {
                                folder = folder, 
                                file = file, 
                                dateString = dateString,
                                sortDate=dt.year..dt.month..dt.day..dt.hour..dt.min..dt.sec
                            }
                            --upload_file(folder, file, dateString)
                        end
                    else
                        print("Skipping ".. dateString.." - Blacklisted")
                    end
                else
                    print(path .. " previously uploaded, skipping")
                end

            elseif attr.mode == "directory" then
                print("Entering " .. path)
                walk_directory(path)
            end
        end
    end

    parseThreads = parseThreads-1
    if (parseThreads == 0) then
        --create_thumbs()
        sort_upload_table()
        run_upload()
    end

end

local function create_folders(folder)
    if (#dateTable==0) then
        print("ERROR: DID NOT FIND ANY DATES!")
        return
    end
    for i = 1, #dateTable do
        local message = "d="..dateTable[i]
        local headers = { 
            ["Content-Length"] = string.len(message), 
            ["Content-Type"] = "application/x-www-form-urlencoded"
        }
        local b, c, h = fa.request{
            url = "http://"..server.."/"..serverDir.."/cd.php",
            method = "POST",
            headers = headers,
            body = message
        }

        if (b~=nil) then
            b = string.gsub(b, "\n", "")
            b = string.gsub(b, "\r", "")
        end

        if (b and b == "success") then
            print("SUCCESS FROM SERVER FOR FOLDER:"..dateTable[i].."<<")
        else
            print("FAILED RESPONSE FROM SERVER FOR FOLDER:"..dateTable[i].."<<")
            print("ADDING TO BLACKLIST")
            blackList[#blackList+1] = dateTable[i]
        end
    end
    print("OK FTP Starting...")
    walk_directory(folder)
end


local function get_folders(folder)
    parseThreads = parseThreads+1
    local tableCount = 1
    --Get the date range from the file
    for file in lfs.dir(folder) do  

        local path = folder .. "/" .. file
        local skip = string.sub( file, 1, 2 ) == "._"
        local attr = lfs.attributes(path)

        if (not skip) then
            if (attr.mode == "file") then
                print( "Datesearch Found "..attr.mode..": " .. path )
                local dateString = ""
                if (attr.modification~=nil) then
                    local dt = FatDateTimeToTable(attr.modification)
                    dateString = dt.day.."-"..dt.month.."-"..dt.year
                end
                if (not exists_in_table(dateTable, dateString)) then
                    dateTable[#dateTable+1] = dateString
                end
            elseif attr.mode == "directory" then
                print("Datesearch Entering " .. path)
                get_folders(path)
            end
        end
    end

    parseThreads = parseThreads-1
    if (parseThreads == 0) then
        create_folders(folder)
    end
end

-- wait for wifi to connect
while string.sub(fa.ReadStatusReg(),13,13) ~= "a" do
    print("Wifi not connected. Waiting...")
    sleep(1000)
end

sleep(30*1000)
get_folders(folder)