tvo, backing-up is an EMR-related issue that seems to evoke a broad range of strong opinions. I spent the first year or so using AC with a suboptimal back-up "regimen", but I used the time to listen to Bert and others (at the 2010 ACUC), reading here (some LONG threads), and elsewhere about the options. I think what I do works for me, and it is posted below. You may want something completely different.
Some people think that this is an overblown issue (see Stephen above), but others want back-ups in multiple physical places, on-line, with many accessible versions over a period of weeks or months, able to withstand computer crashes, power outages, flood, fire, and nuclear holocaust. Again, you need to decide what is comfortable for you. Your hardware is a factor as well; if you have server(s) with RAID you will look at this differently than if you just have a simple peer-to-peer set-up like we do. The speed of your internet connection may also be a factor.
On the other hand everyone MUST have some back-up system. Decide what works for your comfort level.
Then consider the following:
What to back-up: For AC this really means two things: the AC enc file (which is small) and the Imported Items file (which might be much bigger; ours is 27Gb). That makes online back-up of your II file much more challenging. You also want to make a back-up (an image) of your entire drive.
Where to back-up: We want copies of the above on at least two computers in our office, on a secure external hard drive separate from the server, and online.
For hardware, we purchased an ioSafe Solo Pro hard drive which is water-proof and fire-proof. No additional software is needed to create an image of our server hard drive, and periodic copies of the imported items folder. Older versions of the II folder are automatically deleted, so we keep about 10 of them. I think the likelihood of corruption of those files is relatively small and I see little need to keep months of old versions. AC automatically backs-up to this drive. So in the office, rapidly accessible is a full version of the server, as well as of AC and its data. In case of drive or other issues on the server, this is fairly quickly copied and another computer in the office is quickly put into action. If there is a fire or flood, it may take some time to get to the drive, but the data is there if that proves to be the fastest way to get up and running.
We use the back-up that comes with AC to make a copy to two places. We do NOT use it to back up II. One copy goes to another computer on the network; if the server goes down that version of the program is available immediately. Another goes to the external hard drive above. Both are set to automatically delete files after 10 days. Yes, AC automatically backs-up a copy of the enc file right to the AC folder; no one seems to know why it does that or what value it provides. If you choose, you can also use AC to do a daily back-up to a zip drive you carry home with you at night.
Finally, we use Jungle Disk. This has a somewhat annoying interface/access, but once set-up works well. We do a daily back-up of the enc file, and of the II file. The beauty of this program (aside from the fact that it is pretty cheap) is that it uses a process called deduplication. I do not understand how it works at all (Bert, feel free to jump in) but it allows repeated back-up of large files by only identifying changed files, leading to a major speed-up of the process. So it took 9 hours to back-up my 27Gb II file the first time, but daily runs are only two minutes (less than the full run of the far smaller enc files).
