Hello mithilander!
I've just come in the forum for the day, and I read the tread so far. I see, though, most of what I would suggest for the moment has been done.

Thank you all!
So, mithilander, please download the WD DLG software, as by derekimo's suggestion, and post a screenshot of what the software sees. Is the WD AV there still as 26GB? Then, please perform the Quick and Extended tests. Also, please check the jumper settings, too, as by Jumanji's suggestion, because this is an older type of HDD.
How long have you had the drive? Have you used it before, was it on the same system?
What is your motherboard? Please post the model, I would like to check the controllers.
I see you have a lot of physical drives. How does the drive seem in BIOS? The screenshot of your Disk Management isn't full, I see another Disk 7 below, is it a HDD/SSD as well? If it is, that makes for 8 physical devices connected.
After you do and post the test results, I would suggest getting a new IDE data cable/ribbon, and try connecting the drive to another system. Also, switch ports with another drive on your system, to see if this isn't a motherboard issue.
Good luck and post back!
CK_WD