bool DeviceData::devicedataupdated()
{
    QFile deviceListFile(_JsonFile);

    if( !deviceListFile.open( QIODevice::ReadOnly | QIODevice::Text ) )
    {
        return false;
    }

    QJsonDocument jsonOrg = QJsonDocument::fromJson( deviceListFile.readAll() );

    deviceListFile.close();

    if ( deviceListFile.open( QIODevice::WriteOnly | QIODevice::Text ) )
    {
        QJsonObject newDevice;

        newDevice.insert( "IPA",      "newdevice" );
        newDevice.insert( "Icon",     "fa_bolt" );
        newDevice.insert( "Name",     "New Device" );
        newDevice.insert( "Status",   "online" );
        newDevice.insert( "Type",     "Strip" );
        newDevice.insert( "UVLights", "false" );

        auto top = jsonOrg.object();

        auto devices = top.value( "Device" ).toArray();

        devices.push_back( newDevice );

        top.insert("Device", devices);

        QJsonDocument jsonDoc( top );

        deviceListFile.write( jsonDoc.toJson() );

        deviceListFile.close();
        return true;
    }

    return false;
}

top.insert("Device", devices);

    if ( deviceListFile.open( QIODevice::WriteOnly | QIODevice::Text ) )
    {
        QJsonObject newDevice; // projectDetails

        newDevice.insert("IPA",      "newdevice");
        newDevice.insert("Icon",     "fa_bolt");
        newDevice.insert("Name",     "New Device");
        newDevice.insert("Status",   "online");
        newDevice.insert("Type",     "Strip");
        newDevice.insert("UVLights", "false");

        auto top = jsonOrg.object();

        QJsonArray devices = top.value("Device").toArray();

        devices.push_back(newDevice);

        QJsonDocument jsonDoc( top );

        deviceListFile.write(jsonDoc.toJson());

        deviceListFile.close();
        return true;
    }

does nothing! like the file remains as it was :/ no new entry was added when running the code nor any error!

auto top = jsonOrg.object();
auto devices = top.value("Device").toArray();
[...]
QJsonDocument jsonDoc( top );

With that it should work.

    if ( deviceListFile.open( QIODevice::WriteOnly | QIODevice::Text ) )
    {
        QJsonObject newDevice;

        newDevice.insert("IPA",      "newdevice");
        newDevice.insert("Icon",     "fa_bolt");
        newDevice.insert("Name",     "New Device");
        newDevice.insert("Status",   "online");
        newDevice.insert("Type",     "Strip");
        newDevice.insert("UVLights", "false");

        QJsonArray devices = jsonOrg.array();

        devices.push_back(newDevice);

        QJsonDocument jsonDoc( devices );

        QJsonObject obj = jsonDoc.object();

        deviceListFile.write(jsonDoc.toJson());

        deviceListFile.close();
        return true;
    }

File Readl All :  QJsonDocument({"Device":[{"IPA":"studiolights","Icon":"fa_bolt","Name":"Studio Lights","Status":"online","Type":"Strip","UVLights":"false"}]})
Error :  "no error occurred" 0

deviceListFile.readAll();

Except: make sure you call readAll() ONCE - store it in a variable, then print with qDebug(), then pass to the logic below it.

If you call it twice, the second call will likely be empty.

qDebug() << "File Readl All : " << deviceListFile.readAll();
qDebug() << "Error : " << error->errorString() << error->offset;

File Readl All :  ""
Error :  "no error occurred" 0

it just comes up empty :S

Check what readAll() actually returns. Plus add error checking to JSON document (add and check QJsonParseError to your fromJson() call).

QJsonArray devices = jsonOrg.array();

Add a log here (or look it up in debugging session) to see if there is actually any data read from the file.

hmmm the print out comes up empty :S infact it does not print at all

        for(int i = 0; i < devices.size(); i++)
            qDebug() << "Json Original : " << devices[i];

@sierdzio wait QFile::ReadOnly & QFile::Text i thought it was QIODevice::ReadOnly and QIODevice::Text

Doesn't matter it's the same enum and the values are the same. I prefer using QFile because it's shorter and more consistent.

if( !deviceListFile.open( QIODevice::ReadOnly ) )
{

You are opening the file without text mode.

if( !deviceListFile.open( QFile::ReadOnly | QFile::Text ) )

QJsonArray devices = jsonOrg.array();

Add a log here (or look it up in debugging session) to see if there is actually any data read from the file.