Q: How to get an iPod shuffle into disk mode

I've found an old iPod shuffle, I think it's the first one that had the clip, so 2nd Gen? When I plug it in a red light appears on the iPod but my mac shows no signs of picking it up. It got picked up the other day but it came up as a memory stick rather than an iPod, so I suspect it's been reformatted and can't be turned back, but why doesn't it get picked up at all?