Thanks. Too bad the default kernel will only load the ub (read: broken and slow-ass) driver. I'm actually not going to recompile anything.
Anyway, I got it to work. I downloaded and booted into the latest Knoppix (3.9) and tried the same thing. Knoppix also uses the ub driver (whyyyy?) so unmounting is really really slow. But, it seems to have a better mkdosfs:
mkdosfs 2.11 (12 Mar 2005)
unable to get drive geometry, using default 255/63
Notice the "using default bla". Very nice. Works like a charm.
Does anyone know if the 2.4 slack kernel also uses the ub driver?
edit. Slack 10.1 stock kernel (2.4.29) uses the usb-storage driver. Now I understand why it's the default kernel, stuff like this.