There are several software utilities that will do exactly what you want.
The problem is a few will only copy that file when it is not being used. This is only a problem for companies who have users accessing data 24 hours a day every day. (sales departments) However there are a few others that will back up anything on the fly even when that file is being used. Either way its your choice.
There are RAID or there are software solutions. Those who want to spend money and show off, or has a high fear of a data loss use system techniques that are practical for such. Sensitive data or BIG outfits with many users accessing lots of data in a data base need redundancy. This type of user gets complicated RAID systems..
Those who just want it to work and be practical and have no TRUE need for RAID. They do not have a fear of data loss and do not use multi user systems accessing big relational or SQL data base files will use software.
I knew of a few websites using RAID. When they went down they stayed down until some IT tech could remount some hard drive.
I also know of a few websites that operated on regular systems. When they lost a hard drive they were back up within minutes because the tech simply had to change the cable and reboot.
I use software. I have never needed or saw the need for a RAID system. Most of us have no need for RAID. In my opinion. Software will do us just fine.
I personally used genie server back up software. It will back anything to anywhere on any schedule. I had a central system and three other desktop locations. Each one would back up to another desktop. And the central would back up to a second drive. EAch weekend it would back up to a DVD. SO there was never a chance to lose anything. All of his was so transparent I would sometimes check just to make sure it was working.
There was always at least three backups of DATA files. I don't mean whole systems. Just data. This prevented huge unneeded files.
The software would examine all the data files each night and backup only those files that had a change made to it. This is what I used for over 15 years even in the computer shop.
IF you wish you can set up the software to back up changed files every hour if so needed. But, that is over kill.
In summary, 96% of us would do fine to have backup solutions based on software. Very few of us need anything more because how many of us make more than a few new graphic files a day.
How about computerized point of Sale? Yes, that is important. If you do 50 sales a day then you are looking at some kind of daily or hourly back up system.
What is the trade off?
If you have a high fear of data loss or down time. And such down time would cost you lots of money,, And you have users accessing data 24 hours a day. And you do not mind the extra work it takes to maintain the system,, And have the experience and knowledge, and you have the bux,, then use RAID
If you are a regular user such as 96% of us here, and you do not need to run your system 24 hours a day, and you do not have a high fear of data loss, and a couple of hours of downtime will not cost you thousands and do not have an ego driven need to have the biggest then backup software is fine.
I have never ever lost a single file of data..
Thats my personal opinion.