Backup ext to ext then from pc

[zaob]

I have an older external hard drive that I use to back up from the computer plus extra stuff through the years. Got new external hard drive. Would like to copy the old hard drive as is to the new one, and then backup the PC on top of what was copied from the old hard drive, i.e without creating separate backups for the old drive and the PC. Basically I’d like the new hard drive to work exactly from where the old hard drive works. Thanks