I am trying to work out the logical device name on a HP 9000 system running HP-UX 9.1.

I see that ioscan-fnC disk looks like the command on later versions of HP-UX unfortunately it seems the n option is not available on 9.1. The output from ioscan -fC disk is:-

Class H/W Path Driver H/W Status S/W Status
============================================
disk 14.0 scsi ok(0x202) ok
disk 14.6 scsi ok(0x202) ok

I know what the root disk is from:-
#bdf
Filesystem kbytes used avail capacity Mounted on
/dev/dsk/cEd0s0 868974 375390 406686 48% /


I thought listing the devices may give me a clue, they don't:-
# ls -l /dev/rdsk
total 0
crw-r----- 1 root sys 47 0x0e0000 Jan 31 2000 cEd0s0
crw-r----- 1 root sys 47 0x0e0100 Sep 18 2003 cEd1s0
crw-r----- 1 root sys 47 0x0e0200 Sep 18 2003 cEd2s0
crw-r----- 1 root sys 47 0x0e0300 Sep 18 2003 cEd3s0
crw-r----- 1 root sys 47 0x0e0400 Jun 15 2006 cEd4s0
crw-r----- 1 root sys 47 0x0e0500 Jun 16 2006 cEd5s0
crw-r----- 1 root sys 47 0x0e0600 Jun 15 2006 cEd6s0


My object is to clone the disks but until I can confirm the logical device name I can't be sure. I ran mediainit on cEd6s0 as I am sure that is not the device but it just sat there for a good time (until I got bored waiting for it to finish, about 15 mins) doing not a great deal.

Why are you sure that cEd6s0 is not the device? I would suspect that it is. The paths are 14.0 and 14.6. Card 14 is cE because 14 in hex is E. Then the disk number is either 0 or 6. But just do:
dd if=/dev/rdsk/cEdxs0 bs=1024 of=/dev/null
for each device and while it is running see which disk is being accessed by looking at the lights on the drive. As you hit each drive with this command, the lights should start blinking.

I was just guessing, I think you maybe right and 6 is the disk. I ran diskinfo on it and got:-
# diskinfo /dev/rdsk/cEd6s0
diskinfo: can't DIOC_CAPACITY/dev/rdsk/cEd6s0: No such device or address

But on the other devices just got no such device or address I just need to confirm the DISKTYPE on the disktab file now I think.

Could it be you have a SDS disk array managed by the command sdsadmin?