Pdp11 2.11bsd floppy direct access

After taking a further look, the DKF_WLABEL flag that is checked by the kernel mode routine partition_check() in src/sys/sys/ufs_disksubr.c is set or cleared by the routine ioctldisklabel() in the same source file when handling the DIOCWLABEL ioctl. The DIOCWLABEL ioctl is issued by the user mode disklabel utility in the source file src/bin/disklabel/disklabel.c It appears that the usage "disklabel -N disk" should disable write access to the disk label area and the usage "disklabel -W disk" should enable write access to the disk label area, where I think disk should be the device /dev/rra8a you were using in your case. So maybe all you need to do is use disklabel -W before trying to dd to the rx33 disk. I don't have a system running at the moment to give that a try myself.