When you say the disc is not recognised do you mean that it is not seen in My Computer but recognised in device manager or it is not seen in either of those two?

Its not seen in either of these. Its just dead....

You might want to try a different USB cable.  Dell systems seem to have issues with current loss on the cable which can cause the enclosure to not even get enough power to spin-up the drive.  I know that using a 4" USB cable,  a co-worker was able to use one of those drives,  but using a 36" USB cable,  he wasn't.  We ordered several 12" cables on-line and that has resolved his issues...

I have tried a 12 " cable but that didnt make any difference. My existing cable worked fine with the other D505.