Thanks ram but externals are not an option (I don't like the warranty conditions)
The laptop I'm using is a Clevo P151HM1 with the latest Renesas USB 3.0 and Intel RST drivers. And that Penta enclosure looks sick but I can't afford it.
Anyway today I connected the 3TB GPT drive to a PC internally and after writing 2.5TB of data to, rebooting and unplugging the SATA cable several times there was no corruption, further leading me to suspect it's a crappy USB chipset. I'll try a slightly more expensive Mediasonic enclosure and if that gives me trouble I'll step it to a Silverstone. Yay for Shoprunner's free shipping and returns.