So I downloaded the miniroot.fs.gz from the 200206130000/sparc64

# gzcat /NetBSD/miniroot.fs.gz | dd bs=4k of=/dev/rsd0b conv=sync              
1792+0 records in
1792+0 records out
7340032 bytes transferred in 25.720 secs (285382 bytes/sec)
# halt
..

Sun Ultra 1 SBus (UltraSPARC 167MHz), No Keyboard
OpenBoot 3.11, 256 MB memory installed, Serial #8438477.
Ethernet address 8:0:20:80:c2:cd, Host ID: 8080c2cd.

Rebooting with command: boot disk:b netbsd -s                         
Boot device: /sbus/espdma@e,8400000/esp@e,8800000/sd@0,0:b  File and args: netbsd -s
NetBSD IEEE 1275 Bootblock
.
Fast Data Access MMU Miss
ok 

Note that the sd0b is not in use so should be no risk that anything
wrote to it after the dd.

Please cc me on replies - I'm not yet on this list.

Thanks
--sjg