Portable drives are probably cheaper because they're using older 15mm drives - I've been waiting ages for them to go above 1TB with drives that actually fit.

Is there something batch-specific that makes a drive less reliable?

If there was some cost cutting factor involved then I could understand it, but otherwise my assumption would be that external/consumer drives are the ones more likely to have people bothering to claim on warranty so it'd be the other way round?

Ah, that's the wrong data sheet - the extra 6 grams on the LX001 is because that's the "hybrid" drive (FireCuda!!!!!), so it'll be the weight of the SD card or whatever it is that allows them to do their marketing bollocks. £98.50 for that extra solid magic which may or not make a few things a slight bit faster in some situations.

The other two (LM015 Barracuda/LM007 Mobile HDD) are identical weight

I got an adata 1TB usb3 about a year ago for a very good price which I promptly reformatted to ext4. No complaints thus far. It does need a powered hub for my home pc to recognize it.