To synchronize files, between two folders (or the whole partition), you can use https://www.freefilesync.org/
You can simply copy-paste your files first and then run FreeFileSync periodically between pairs of folders. It is easy to use and powerful ; just take care not doing mistakes, but this is valid with every software.
To create a low level clone ─ which mean that you can both access your files from the external drive, as well as start the OS from the clone exactly like thus was the original drive, whish is especially useful in case of an OS failure ─ you could use Acronis True Image.
Take care that Acronis can extend the partition to thus of the external drive.
For instance, if your internal hard drive has a capacity of 250 GB and your external drive a capacity of 500 GB, the clone would be 500 GB, forcing you to use a 500 GB medium as target when you want to clone back. So, when performing a clone with Acronis, I would advise using a target drive with capacity no larger than thus of the source drive. Also take care that two drives of the "same" size (eg. 250 GB) may have a slightly different number of available sectors, depending on the model. The best would be using a drive similar to the one present in your computer.
To create a low level clone, you can also use Linux
dd command or ─ better ─, Linux's
ddrescue which will allow you to log errors to a text file.
You can run Linux from a live CD and write the log file to an USB key that you mount.
ddrescueis present on SystemRescueCD.
For more comfort in case of a regular use, you can install Linux on a hard drive or SSD of small capacity, install
ddrescue, boot from this drive and write the log file directly on it. If you give a descriptive name with date to your log files, you can keep track of your backups history.
To copy specific folders from the console/terminal, also look at Windows
xcopycommand and Linux