Problem with HD recognition

I'm a new member of this forum.

I have read the various threads similar to mine but none is axactly the same!

Earlier this year I had a motherboard "blow up" on a Win 7 PC. I'm up and running with a brand new desktop machine now - still with Win 7. I removed the twin HDs from the old machine when it happened and using my laptop initially managed to salvage some data and programs. When the new Desktop machine arrived with 2TB space, I recovered more from both HDS.

Now I've decided it's time to finally clean the two old disks and use them as storage or backup in external drives. However, one of them now isn't recognised by either my desktop or laptop PCs, whereas it was before! Both were read several times some months ago as I've been gradually retrieving data from them as time allows. Over the past two days I've extratced around 400 GB of stuff from the other drive with no bother.

I'm fairly confident that there won't be anything vital left on the second drive at this stage, but I can't even format it to start again when it isn't recognised! I have tried most of the "solutions" suggested to others with similar problems. The two housings in which I have put them each has its own power supply and the same USB port reads one but not the other!

Any solutions would be appreciated.
Thank you.
Kinveachy