Hi there,
I would like to let you know that I am a WD representative and I am not going to discuss if our brand is better than the competitive.
Generally speaking, it is always a good decision to get an external drive to backup your data. I wouldn't say that the drive size matters in case of heating, noise and performance. It is more a HDD type matter.
If I were you, I wouldn't use internal as an external. I will tell you why. The concept of doing a backup is to secure your data and using an internal drive in an unusual environment is not the best decision. If you put the drive inside the desktop, it is not a secure backup anymore. I would recommend you to think about external drive that comes with its own enclosure, but, of course, if you insist, USB to SATA connection may do the job for you. Have in mind that using and internal HDD as an external with a third party enclosure may not be fully supported. Check the link below.
I see that you need to know the drives inside the device and not to be dependent of the encryption. You can always get an external without the encryption, like the WD Elements or WD Elements portable. As for the drives inside, check the link below for the official WD position.
The truth is that, like other mechanical devices, you can never be sure when the drive is going to fail. You can only make sure to handle the drive properly and protect your data. Using two externals is not unusual decision and I would say that, if you have the option, go for it. Do scheduled backups and store your devices separately. If anything goes wrong, do a RMA and rebuild.
Talking about WD drives, the WD Black is a great drive, when talking about performance and warranty. The truth is that no drive can be fail proofed. I can give you an advice: if you want to benefit the better warranty, go for the Black drive, but you will not experience the better performance when talking about a backup drive. I would use a Green or a Blue drive for this purpose. That is why you may find one of those in a desktop external.
And finally: You can never know when a Data Recovery will be easier. If you do not have backup and the drive fails, the best scenario is not to try to save the data yourself, but to contact a Data Recovery Company, because your actions may make recovery harder. In other words, do backups and don't be dependent of such companies
http://wdc.custhelp.com/app/answers/detail/a_id/296 – Some information about installing SATA Drive in an external enclosure
http://wdc.custhelp.com/app/answers/detail/a_id/1704 - What drive is inside a portable device
Hope this helped