So, my ancient copy of Leadtek’s Winfast PVR2 usually works fine, then one day it decides it’s going to split recordings at 1.99GB no matter what you ask it to do.
Reading some ancient forum posts, someone else mentioned that it will do this if it thinks it might be on a FAT16 (or FAT32?) drive. It isn’t in my case, but still it’s been known randomly to switch into this state of mind.
The solution is unknown, but switching to another physical drive seems to resolve it.