df -k would only show you the device if it was currently mounted. Yes, the format command is probably the way to go. Did you remember to do a boot -r (or touch /reconfigure then init 6) after you added the new disk? If not, run devfsadm. Then the appropriate device entries will be created and you should be able to see the disk when you run format. You can use dd, or even ufsdump|ufsrestore to transfer your data. You're best to umount the source disk before you copy - if anything is writing to the filesystem whilst you're copying it's contents you could end up with corrupt data.

Cheers
ZB