You need msdos fs support in the kernel, as the error message says. Add this support as a module so you won't have to reboot to enable it.
Chandran Hank Fay wrote on Sat, Sep 05, 1998 at 03:37:27PM -0400.. > I rebuilt my kernel (Hamm) for firewalling, and included the floppy support > option. However, since then I can't mount the floppy drive. I see it > recognized (fd0) at boot up > > I'm told that fs type msdos is not supported by the kernel. When I try vfat > the same. When I try ext2, I'm told it is "not a valid block device." > > Hank