below..

On Mon, Jul 27, 2020 at 11:02 PM Will Senn <will.senn@gmail.com> wrote:
On 7/27/20 8:14 PM, Clem Cole wrote:
on v7:  dd if=/dev/rrkN of=ucbtar bs=1        <-- this should pull the binary with the padded last block in
ouch my screw up...  should be 1b  

Your instructions held up until I tried to read from the rk device - here's my attach:

in simh:
set rk0 rk05
att rk0 ucbtar
This is what I have:
; RK05 data disks (4 drives)...
;; SET RK ENABLE
SET RK0 WRITEENABLED
SET RK1 WRITEENABLED
SET RK2 WRITEENABLED
SET RK3 WRITEENABLED
SET RK4 DISABLED
SET RK5 DISABLED
SET RK6 DISABLED
SET RK7 DISABLED
ATTACH RK0 ./scratch.rk05
SHOW RK
 

then in v7 the rk's aren't in /dev, so:
there is a makefile in /dev that creates them.  Take a look and see what its doing. 

looked in c.c, rk is major dev 0, and rrk is dev 9, so...
/etc/mknod /dev/rk0 b 0 0
/etc/mknod /dev/rrk0 c 9 0
chmod 640 /dev/*rk*

and
dd if=/dev/rrk0 of=ucbtar bs=1
bs=1b   sorry -- my error, but I don't think that's the issue. 
try this for grins:  dd if=/dev/rrk0 of=/dev/null bs=1b

cannot open: /dev/rrk0
Hmm... be good to see what the errno value is, i.e. why?
Maybe a small C program that tries to open /dev/rrk0 and prints out errno on failure.