Let me analyze this a little more. Long ago, I read W. Curtis Preston's book "Unix Backup and Recovery", and still have it on the shelf. That book is old -- early 1990s and out of print. Preston's setting is a data center. He discussed writing backup data to various forms of tape media. Isn't that an example of a host machine writing to its own physical devices? Those hosts could certainly suffer hardware failure too, and no doubt they did, but the threat of hardware failure wasn't enough to cancel out the benefit of doing regular backups. He also discussed using remote copy to send data to another host, but noted the protocol was insecure. I seem to recall he was definitely more focused on tape backups and on testing the actual backups to make sure data could be restored when an emergency struck.
Mainframe systems have been virtualized for a long time now, and are normally connected to numerous devices, and these virtual hosts write backup data to connected devices. Again, hardware failure, while always possible, isn't a sufficient reason for not doing a backup using the resources available and on a regular schedule.
Backup solutions aimed at Tier 3 end users likewise focus on having the user's current system(s) send data to some type of connected media or increasingly to a remote host somewhere in the cloud. For example Dropbox. External drives connected to the user's system are very popular choices. The user's system can certainly crash due to hardware failure. That doesn't preclude doing a backup, however.
What these systems seem to do is focus on writing data on a regularly scheduled basis to removable media, whether this is tape media or a MyBook. Or send the data to some entity in the "Cloud", a remote host with storage capacity.
That is the secret. The bold. Tape backups is totally different from hard disks for multiple reasons. Most notably that they have their own power supply, they are usually physically removed after the backup is completed, and they arre often proprietary enough that you can't execute a single command that serves various useful functions(dd for example) and destroy the data on them. Not to mention the fact that for the quantity of data you backup a Tape drive is virtually out of the question.
Tapes are not readily accessible to the OS with millisecond notice. Often with a tape, if you "update" a file, the tape simply write another copy of the file to tape wherever the next free space is. If you go to do a recovery you'll see multiple copies on the same tape.
While tape stores data just like a hard drive, its inner workings, its functions and operation are very different internally. Often SATA drives need zero drivers to work. The motherboard drivers are common and generic and will "just work" with most OSes that are in use. Tape is often totally different. If you are using a single tape drive, it may simply be plug and play, but backing up data will certainly take someone swapping tapes out VERY regularly. If a tape is destroyed you just go to the previous days' tape. With multiple tape decks and other big multi-tape setups they can't access all the tapes at once and wipe them all out. Typically the tape machine is sent a command "load tape 54", then "mount tape 54", then "write this data". You must wait for the first command to finish before you send the second. That is not true with SATA(obviously to anyone that understands NCQ).
So no, tape is not the same as hard disks.
Also, you said that your storage media is portable. I'm sorry, but I disagree. Just because it's hot swappable and in a tray doesn't make it "portable". I'd consider tapes portable. Hard drives much less so. Every USB hard drive I have bought in the last 6-8 years has even come with a big red sheet of paper when you open the box that says "I am not portable.. do not use me as such". Even ones with laptop drives! The only exception was a Kingston HyperX(SSD).
While your logic is sound, it's also missing a lot of the bigger picture. You are welcome to sleep well at night with your backups. I know I wouldn't. I've seen virtual machines go down and take all of the data they had attached with them to data-heaven. I virtualize FreeNAS only for experimenting and testing things. The only files in my virtual machine are a backup of my HTPC, laptop, and desktop. The virtual machine is actually located on my file server. Notice there is no physicality between my backups and the computers they backup. I've never had to use the backups, and I've occasionally crashed FreeNAS pretty badly(woohoo for snapshots). But I'd never think of setting up a system with the original and backups always online and always available to the OS like you do.
Bigger picture, you are talking about virtualizing your backups to save you about $500-800(perhaps less depending) by not building another physical system. That tells me that your data is worth less than $800. If your data was worth $250k, you'd be paying money for offsite backups with some large company or otherwise dedicating time to asking a friend to let you borrow a drawer in his bedroom to store AES-256 bit encrypted hard drives. It's always easy to see what someone's data is worth by looking at how much they're willing to spend. When they won't spend more than $x, you know that's what they value their data at. I've seen computer virus wipe out data on internal and externally connected drives simultaneously. Risk is the name of the game. If you are okay with that risk, than a USB drive is for you. If you've had it happen to you before and you think its likely to happen again, you'll DEFINITELY consider another form of backup.
If you sleep better at night because of what you call backups, then great. If I sleep better at night because of what I call backups, then great. When you lose your data I'll feel bad for you, but I won't miss your data anywhere near as much as you will. The reverse is true if I lose my data. It's as simple as that.
You've chosen to accept certain risks for your configuration. They are either acceptable risks for you or you don't know enough to realize the risks(which is always universally bad). If you accept the risks versus the cost, then great. I had a tornado go down my street in 2010. It scared the crap out of me and made me realize that if my house was leveled(my neighbor's was) that I would REALLY miss some of my data. Pictures of my time in the Navy and going all over the world would be gone forever. I bought a $150 2TB drive and I backed up all of the data that would devastate me if I lost it, including scans of important documents and pictures of my entertainment center and what-not. I then had a friend 25 miles away keep it in a shoebox in his basement. I will tell you that when you lose all of your data you are heartbroken beyond explanation. To me, spending that $150 made me feel much better after that brush with the tornado.
