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If you are using a CD-RW (or have one handy), try putting an MP3 file on the disc to see if it has problems recognising these discs. If you get 'Invalid Disc' for a CD-RW (or CD-R) containing MP3 tracks, then there is something wrong with the drive itself. In this case, the drive will need to cleaned, such as using a laser-lens cleaning disc. See ths tips page for more info on choosing a cleaning disc.
Generally if the DVD recorder does not recognise the firmware on the disc (such as for a different model), it will display 'Data Disc' on the screen. The 'Invalid Disc' generally only ever appears if there is something physically wrong with the disc or drive, such as cracked disc, loading a DVD-RAM disc, pick-up issues, etc.
The correct approach to installing firmware is to put the firmware as a file on the disc, just as you would writing documents to CD. If you succeed in getting it to play an MP3 from CD-RW, use this disc for the firmware update process, as this would rule out a bad disc or read issue.