I installed ReactOS on third partition(primary). First(also primary) is ext4 partition with grub and second partition(also primary) is NTFS.
While installing at first stage after copying files setup says:
Setup cannot install the bootloader on your computers harddisk. Please insert a formatted floppy disk in drive A: and press Enter.
When i hide ext4 partition setup asks(must be so in first case) where bootloader must be installed(on harddisk, on floppy disk, etc)
So the bug is that setup gives error about impossibility of bootloader installation without asking about that installation.