Originally Posted by itimpi
I think the OP was using Linux so this would not happen
I would have thought a new debug log with the device connected is likely to be to be most helpful thing for diagnosing the problem.
Okay, just a quick update. I reverted back to stock ROM on one of the Android devices and when I launched Calibre, Voila! It was detected. I was able to transfer books and do all the usual stuff. The thing is, I normally don't use this device for reading as it has a really very small screen.
I also noticed that this device showed up in the RhythmBox Player (which is default on Ubuntu) on the stock ROM. The other phone was always on stock; blame that on the lack of developers for it. Anyway, I can't seem to think of any plausible reason why Calibre detects one and doesn't the other. I'm going to take a look at the Udev rule shortly, as someone above suggested.
Meanwhile, I'm using the Calibre Companion app on the Play Store to transfer books to the other Android device.