First, I'm not a Solaris user so this may not apply.
What kind of hardware are you dealing with? Particularly hard drives and hard drive controllers. To write stuff to an external drive you need to have it connected to a controller and the operating system needs to be aware of the drive and have it attached to the filesystems.
In the SCO world, using SCSI hard drives for the system, it is a matter of using mkdev to create the mount points for hardware then rebooting to get the hardware stuff in place, use mkdev again to condition the hard drive, mkdev again for filesystem creation and let it mount the newly created filesystem. Then it is a matter of copying stuff to the new location. In this regard the external hard drive is no different from an internal drive.
Moving the hard drive off the system is an issue of disconnecting the filesystem and taking it away via umount.
Putting the hard drive back on the system uses mount.
You'll probably get more specific help from a Solaris user.