1)Availability
2)Price
3)Drive Capacity
4)Portability (ie size)
The answer seemed to be a Western Digital Passport 500Gb which I could get for a reasonable price the following day. I didn't bother googling this device because, I reasoned a hard disk is a hard disk is a hard disk right? Well normally yes, but in this case - no. I knew it would be partitioned with ntfs and I knew it had a bit of windows type bloatware on it (backup and encryption software I think) but most things you buy have this kind of stuff on them nowadays and a quick format into a linux file system and it is all gone.
So I picked it up the next day formatted it, partitioned it and transferred all my data onto it. Job done. For a while I was quite pleased with the purchase.
That feeling melted away as soon as I tried to reboot - the boot process stopped stone dead as soon as the disk was recognised. If I unplugged the disk everything worked normally again.
Puzzled by this I started to do what I should have done before I bought it ie. googled around. The net is awash with people complaining about this device and there are the usual confusing array of 'solutions' to make it work properly. Having got a flavour for what other people had tried I started to investigate the problem myself. These are the things I tried:
Remove the device completely from the bios boot order - no effect.
Move the main hard drive to the top of the bios boot order - no effect.
Erase the mbr with dd - no effect
Run Test Disk on the device to look for hidden partitions - none found.
Do a low level format of the device with dd - no effect.
These last two took several hours and gave me more time to google the subject. It appears that the device has a virtual cd device loaded into its firmware and this is completely unremoveable. I even used my wife's Win 7 computer to flash the firmware it made no difference to the device though. The WD support site lists a procedure which is supposed to switch off the vcd (though it doesn't remove it). I tried this but it failed - no 'smartware' device attached (smartware is what it calls its bloatware). I guess that is not surprising since I have formatted it three times!
So basically I am stuck with a device that has to be unplugged every time I reboot. It is not a big deal I guess, but annoying that it doesn't work as it should.
It struck me at the time that this is a little glimpse into the future though with Uefi on the horizon, situations like this are going to be happening every day (uefi codes will apply to hardware devices as well as software in case you didn't know).
So a lesson to learn here, no matter how urgently you think you need something, google the product first with the word 'linux' in the search

By the way I am not looking for answers to this, there arent' any, it is just a warning so that hopefully others don't fall into the same trap.