backup internal/external

First you will need often a larger drive then you have for the OS and other drives internal as well as external. If you have one 500gb OS host drive, a second 500gb internal, and then two 500gb external drives you would likely need a 2tb drive to cover the size of any image created.

The program to handle this would like be Acronis True Image as one of the best paid for programs or one that will include multiple drives into one main image backup. Others like Paragon, Macrium's Reflect, the older Norton Ghost are some of the more popular programs.

With a few those either have free versions as well as paid for or like Paragon offer a free version. Paragon is a free open source program there. Also drive manufacturers like Western Digital will offer a free version of Acronis TI. Seagate also offers the rebranded Disk Wizard(Acronis under a different name) free as well for their customers.

I don't think you said what you meant.

Simultaneous backup of two different drives on the same computer?

The built in option in 7 allows you to create full system image backups with more then just the main drive and 100mb System Reserved partition if seen. You can add an external drive or one in an external enclosure into the same image.

The real problem when using this option for backing up multiple drives is knowing how much is on each one for the actual total as well as having a large capacity destination for the image to be stored on. One example lately was someone wanting to use the 7 option for the 7 host and second storage drive where the data was 1.2tb to create an image to store on a 3rd 1tb drive. He would needed at least a 1.5tb free of all other files to allow room.

The example of four 500gb drives let's say half full for each would 250gb x 4 = 1tb. A 1tb drive sees 920gb available. If you already store files on a 1.5tb drive a 1.2tb would use the rest up in one shot and may not fit. A 2tb drive would then be needed to allow for it.

It's possible to take four drives and create a single image. But is it recommended? NADA! In fact with four 1tb drives here how would I back up all 4 HDs in one image? A network backup location! The size plus any possible corruption of an image too large would be detrimental in the long run.

Here I may end up simply going with a 3tb model for externally backing up files from two internal drives spread across both like some folder also kept on the host drive as well as a system image, VMs, and other things kept on those two drives in order to backup the backup drives themselves. Omitting the 7 host would allow Acronis used here as the alternative would allow the incremental backup of those drives to a larger external HD.