The quick answer is, you buy an external hard drive formatted for use with "Mac" (the filesystem format for Apple vs. Windows is not the same -- while it's trivial to reformat a drive using Apple "Disk Utility" -- since you said you're not a technical person it's probably best to just get a drive pre-formatted for Mac.) Connect it to your Mac and follow the prompts.
The longer answer is below:
Apple's newest Macs use "Thunderbolt" connections and also support USB 3. If your Mac is older then it likely has "Firewire" connections as well as USB 2. USB 2 is fairly slow. Firewire, on the other hand, is very fast. USB 3 is also very fast and Thunderbolt is BLAZINGLY fast. So while any USB 2 connected hard drive will "work", the drive will be faster if it's Firewire (if you have a Firewire equipped Mac) or faster type of connection (such as USB 3 or Thunderbolt -- but ONLY if you have a USB 3 or Thunderbolt equipped Mac wich would need to be a fairly recent model.)
The first time you connect an external hard drive to a Mac, the computer will automatically ask you if you want to use it as a "Time Machine" backup disk. If that's why you bought it, then just click yes and let it go. The first time it runs the backup it WILL take quite a while (possibly several hours). After it finishes, it will begin backing up your computer every hour (as long the computer is powered on.) It auto-manages it's backups... you get hourly backups every hour for the past 1 week. It then starts consolidating the hourly backups into "daily" backups every day for a month. After that it consolidates the daily backups into "weekly" backups and it keeps as many of those as possible until the drive runs out of space. When the drive runs out of space, it deletes the oldest backups to make room for new backups.
It's recommended that the backup hard drive be at least 50% larger than your internal hard drive. E.g. if your internal hard drive is a 500GB drive, then you'd want the backup drive to be 750GB or larger.
Here's an older video of when Apple introduced the technology: http://www.apple.com/findouthow/mac/#timemachinebasics
A quick search (I used Amazon.com) found several external hard drives (these are drives with their own enclosures and cables). The low priced options were about $65 for a 1TB drive. I saw a 2TB drive for $80. And of course... there are more expensive options as well.
You can get these drives in portable formats as well as desktop formats. The portables are usually powered by the bus (USB, Firewire, or Thunderbolt). They are also typically small, compact, and often times they do not have any form of cooling. The desktop formats often have a separate power adapter (so they are not drawing power from your computer to operate) and are usually larger, roomier, and sometimes they have their own cooling fans. The compact portable (e.g. "travel" size) external drives are not intended for extended use. The desktop format external drives typically are intended for extended use (e.g. you could hook them up and let them run all day without a problem.) The truth is the compact portable drives could run all day too... but they'll get hot and this will shorten their life.
It's also possible to buy replacement internal drives that are significantly bigger than the drive in your computer right now. But this all depends on which Mac you have and whether you know a tech-savvy person who knows how to perform the upgrade. Macsales.com specializes in upgrade parts (such as hard drives) for Macs and has a pretty database of all the various models and which ones can take which drives. There are a *few* Mac models (mostly some of the iMac desktops) for which hard drive replacement is actually pretty difficult to do and not recommended.
Keep in mind I've talked about two different reasons for having an external drive and they shouldn't be mixed.
1) External drives to give you more space for your important data (e.g. running out of space on your internal drive) and...
2) External drives to 'backup' your data.
Don't use a drive for 'backps' AND for more space on the same drive. e.g. if you put your iPhoto Library on the external drive and ALSO use the external drive for backups, then if that external drive fails you will have lost your backups AND the primary working iPhoto Library (because you would have migrated that off the internal drive). So now you've lost both copies and it sort of defeats the point of having a 'backup'.
All hard drives (all storage really) eventually fails. So it's just a matter of time before it happens to you. I'm pretty religious about doing backups (but I've been in the industry a long time and have seen many many hard drives fail and many people lose their data for which they have no way to get it back.)
Since I'm a technology guy, my friends ask about those hard drive recovery businesses. Turns out... "I know a guy" (I actually do). But it's not cheap. You're paying for some fairly specialized equipment and a skilled technician who will need to work on your drive and your drive only for probably most of a day. You're paying this employees hourly rate and, since they're in business to make a profit, all the overhead and profit margins loaded into that price tag. Basically he told me the price tag isn't straight forward because lots of factors go into it, but the total is never going to be less than $1000 (and it could be considerably higher). You have to really want your data back to shell over that much money.
This is why I say the $65 external hard drive and the fact that your Mac has built-in backup software (Time Machine -- which is actually exceptionally good software) is really the cheap way to avoid ever being faced with the choice between losing all your photos or paying a pretty hefty bill to hope to get it recovered.
Tim Campbell
5D III, 5D IV, 60Da
