Filesystems, Disks and Memory

I have a SUN 440 running Solaris 8 that is generating funny errors on /dev/dsk/c1t0d0 and I would like to dupe the drive(the non offending drives are removed for this process), swap it with the dupe and reboot. From what I have read, the process seems simple:
dd if=/dev/dsk/c1t0d0 of=/dev/dsk/c1t1d0

Here is my question: Since this is the disk holding the OS, do I need to have the source and or destination mounted/unmounted?

I have tried booting into single user with both drives seated in the chassis and the command simply returns
dd: /dev/dsk/c1t0d0: open: No such file or directory
I have tried booting into single user with both drives out, then re-seating them and get the same error.

Can someone please run down the step-by-step process I would need to accomplish this please? Or point out the one piece I am missing? I watched a co-worker of mine accomplish this on a Trusted Solaris box so I know it is possible and simple...

Thanks in advance,

dd if=/dev/dsk/c1t0d0 of=/dev/dsk/c1t1d0

what is the output of format when you have both disks in?
i would like to know what controller it is on.