Install LM20 in legacy mode (thinkpad T430 BIOS set Both[legacy first]), select 'erase and install' to overwrite a dual boot win10/LM19.3 install in UEFI on sda.
Reboot - why has an 'ubuntu' entry appeared at the top the the BIOS boot list - this is legacy boot, all that should be there is a list of devices??? Select 'ubuntu' and, what a surprise, it won't boot and keeps returning to the BIOS boot list. Select HDD0 and boot into the new LM20.
Why has LM20 created a fat32 partition on a legacy boot? Primary partitions are in short supply on an msdos drive and we don't need the installer un-necessarily using up primary partitions? So what's in that fat32 partition? Reboot into LM19.0 (on another drive) and have a look: Looks like nothing*, so delete partition and reboot into LM20.
It did boot, I didn't time it, but subjectively it seemed to take lot longer than the previous boot into LM20 when the pseudo EFI partition was present. Not about to repeat the experiment, life's too short.
Conclusion - I think the mint/ubuntu team have screwed up the legacy install, probably on the basis that they don't care about legacy anymore.
Recommendation - if you want to install in legacy mode, pre-partition your drive with gparted and use the 'something else' option during install.
The other thing I noticed (this was cinnamon), in nemo:
- LM19.0 - devices pane on the left shows the other drives/partitions in the system, e.g. xyzGB volume (there are three drives in this laptop).
- LM20 - under devices - nothing (but the other drives were visible in 'disks'). Guess this is probably a setting somewhere.
*EDIT - didn't check to see if 'show hidden files' was enabled in nemo. Whoops... EDIT2 - show hidden was enabled.