The subject often comes up on photography forums and a consensus seems to have emerged that goes something like this:
1) Archive: 2 (minimum) external hard disk drives, not normally connected to the computer (for virus, ESD, et al reasons), with at least one located remotely. This is my scenario (my #1 is in a drawer near the home computer, #2 drive is at work). This is for longterm backup and recovery purposes, not for frequently-accessed files. This requires you to determine based on your work habits when a file/folder is moved to the remote drives. Again, an actively working session isn’t moved. The cloud is suitable for this but a bit costly for most folks. HDDs usually get the nod due to value, but some do use thumbdrives, CD/DVD, or even SD storage. My older files are on CD and DVD, but even these were copied to HDD for ease of recovery; in those cases I’ve actually got 4 backups (CD/DVD and HDD at home and work). BTW, I don’t replace an HDD unless I need to. When I need more, I buy a moderately large one and start from there, not moving files from the older drive. The goal isn’t to have all the archived files on a single drive, it’s to have them archived period. If that takes 3-4 separate HDDs all the better actually since a failure won’t take out the entire archive. You have another archive to recover from, right? In fact, I often used old, smaller HDDs that I had replaced in computers. Buy an external drive case for it ($10-20), reformat and, viola! you have a backup drive. At home it’s a 250gb, at work I have a 320gb. When these are full, I’ll buy more.
2) Backup: a pure backup, with ready access, for files presently in process; these you are still working, tweaking, whatever. For me this is on the computer’s internal HDD for the working files and still on the camera’s card for ready back-up, altho in some cases it may be the tablet and card. If you keep them on the card, you might need a few more cards. They’re cheap. I’m using a 16gb and haven’t had an issue yet. Wedding shooters tend to have problems with having enough cards, but not most folks.
Regardless, you must develop a disciplined procedure to cull your shots. Just because storage is cheap, it doesn’t mean that you should save all the real rejects. Yes, you have to define what a cull is, but it makes no sense to spend time and money storing out-of-focus shots, and in our case, the 47 sequence files of the approaching train. If it is something special, I may be rattling off a few dozen at 6+fps. But you can bet that not every one of them is saved. You decide, but pick a few of the best, dump the rest. It isn’t the storing that’s the issue, it’s finding the one you want later.