Do it same as any other hard disk, as being external makes no difference to the software, whichever you use. Some versions that are free will not let you do a source of an external hard disk or a network disk. But there might be a few.
You can pick which drive is the source, which is the destination.
A problem though. Any hardware can fail--one drive is no more secure than another. Sure, having one with less hours on it is better most of the time. But not absolutely dependable. Also the cloning software might fail.
An alternative is a compressed image of the drive or just the files you wish to save. Then put them onto a DVD disk or set of disks. No moving parts.
Also "The Cloud" or several destinations on Internet that offer storage are options. The more copies you have, the safer your data is.
Don't forget to update. That can be done by increments to save time.
Decades ago Corning Glass developed data disks impregnated with magnetic material. Recordings could be made on them. When they were heated, the patterns were frozen. They should last thousands or maybe millions of years.