listDir in sdcard returns empty array

Ionic Native
1 
this.diagnostic.getExternalStorageAuthorizationStatus().then(s => console.log('sss', s))
     this.diagnostic.getExternalSdCardDetails().then(d => {
        let filePath = d.filter(c => c.type === 'root')[0].filePath + '/';
        this.file.listDir(filePath, '').then(folders=> {
        console.log(folders) //[] empty array :( 
       })
    })
})