Looked around and this is quite a common problem, one which I've suffered from on occasion. Basically the data cable connecting the second drive in my machine works itself loose, resulting in the machine not picking it up. Doesn't effect the machine running or anything important, but just about all the games are on it. So a bit annoying.

Other than buying a new cable in the hope it fixes it, are there any quick fixes, like attaching it firmly on with something e.g. tape (not wise perhaps, but I can't think of anything else)?

Firstly what is happening inside your PC case to make it work loose?!

I've had this happen when moving my desktop to another location but it shouldn't be occuring while in situ. How is your cable management? ie are other wires putting tension on your sata cable. Try rearanging things a bit and cable tying up loose cables. Bearing in mind airflow through the case.

I noticed on my new computer the sata cable seems to clip into the mthrbrd and the hd connection was very tight, you could try wedging something down the side when you plug it in. the plastic insulation of an old wire would probably work and you could cut it down to the right size easily.

I must point out it doesnt completely unplug, it just very very slightly moves back so it doesnt connect properly.
And it's the plug that goes in the hard drive, not the other end that goes in the motherboard. I can't feel any undue tugging on it either.

sounds like you just need a bit of friction in your connector to hold it on. Did you try swapping round the ends to see if one holds better than the other. Since buying a new cable isn't guaranteed to fix it i'd be inclined to try adding some packing like in my diagaram where the "red" section is packing.

Glue gun it in