How to read files larger than 100mb

Here's something I wrote recently to copy a big file - could be adapted just for reading:

local __copyBigFile = function( sourcePath, destPath, progressScope )

    local fileSize = LrFileUtils.fileAttributes( sourcePath ).fileSize

    local g
    local s
    local t
    -- local blkSize = 32768 -- typical cluster size on large system or primary data drive.
    local blkSize = 10000000 -- 10MB at a time - lua is fine with big chunks.
    local nBlks = math.ceil( fileSize / blkSize )
    local b
    local x
    g, s = pcall( io.open, sourcePath, 'rb' )
    if not g then return false, s end
    g, t = pcall( io.open, destPath, 'wb' )
    if not g then
        pcall( io.close, s )
        return false, t
    end
    local done = false
    local m = 'unknown error'
    local i = 0
    repeat -- forever - until break
        g, b = pcall( s.read, s, blkSize )
        if not g then
            m = b
            break
        end
        if b then
            g, x = pcall( t.write, t, b )
            if not g then
                m = x
                break
            end
            i = i + 1
            if progressScope then
                progressScope:setPortionComplete( i, nBlks )
            end
            LrTasks.yield()
        else
            g, x = pcall( t.flush, t ) -- close also flushes, but I feel more comfortable pre-flushing and checking -
                -- that way I know if any error is due to writing or closing after written / flushed.
            if not g then
                m = x
                break
            end
            m = '' -- completed sans incident.
            done = true
            break
        end
    until false
    pcall( s.close, s )
    pcall( t.close, t )
    if done then
        return true
    else
        return false, m
    end
       
end

Wilko,

I think trying to read this much in one chunk is a bad idea. Could you explain why you even need to do this?