Turns out after you have written the floppy IMG to a blank uFD or HDD, and it boots and plays the game, then you can partition the drive (I used Linux Mint), format it as FAT32 or NTFS, and use the full drive for mass storage without a conflict.

Same drive still boots to Floppy Bird only, unless you install some other OS or something which overwrites the Floppy Bird bootloader on sector 0, and give up the bird.

Partitioning and formatting alone do not overwrite the MBR, even if it is just a custom floppy boot sector at the time.