I don't know of a way to backup data to an unmounted (unassigned drive letter) drive on a typical PC but one way that you could keep a backup drive physically connected and use it for backups is to manually mount the drive before you back up data to the drive and then dismount the drive upon completion of the backup activity.
That may be an option to consider in situations where one would like to keep a backup drive physically connected to a PC if connectivity to the USB, eSATA, etc, cable[s] aren't easily accessable on the PC.
Another way would be to automate the procedure, with a Task using Windows Task Scheduler, or a script.
For example,. Diskpart could be used to mount and dismount a backup drive.
Something like this would work in Diskpart:
select volume #
select volume #
remove letter x: dismount
I have a "Diskpart Mount" batch file that will do that but I rely on redundant backups for protection against encryption ransomware.
I have one continuously-connected portable USB drive that I use for my automated specific-item backups. That drive is vulnerable to encryption malware so I also backup up the same items to a couple of Flash Drives manually.
I'm using a script to back up with the Flash Drives to eliminate manual drag/drop, copy/paste time and to minimize the time that the Flash Drives are connected to the parent PC.
Just my 2¢ on backup plans but I like to use full-HDD backups in addition to my specific-item backups.
That way, if I'm affected by malware, failed HDD, user mistakes, bad downloads, bad WU's (Windows Updates gone bad), I can either install one of my Clone HDD's or restore from an Image, either of which was done prior to the point at which the PC was affected my malicious events.
My Cloned drives and my Image storage drive are disconnected from my PC for protection against malicious intrusions. When I Clone or Image, I boot my backup software into RAM to process the backups outside of Windows.
As Condobloke mentioned, RollbackRx is a nice option for restoring the HDD to a previous point in time.
My take is that you can't have too many backups. The more full-HDD recovery paths that are available, the more options are present to recover from virtually all undesirable scenarios.