One reason for having two completely independent backup methodologies  is that backup solutions are curiously unreliable. 
This time it's my 3 year old Time Capsule's turn to cause me heartburn. I've gotten the mysterious "-1" error on backup three times in the past 8 weeks (shortly before the Time Capsule Firmware 7.6.1 update and about 4-5 weeks after the 7.6 update) for two separate client machines (Snow Leopard and Lion respectively) for backup stores on the Time Capsule's 500 GB internal drive and its 2TB external drive:
"The backup disk image ... could not be accessed (error -1).
An Apple Discussions post hasn't generated any useful replies ...
... For the 2nd time in 6 weeks my dual core MacBook (10.6.x) fan was racing and activity monitor showed fsck_hfs was running. Again  Time Capsule was stuck trying "Verifying Backup" over WiFi. Even when I connected via ethernet backup would not verify. I had to delete the sparseimage and start again. Once is chance, twice is enemy action. Something is wrong.
My other machines are doing better, but even my new iMac complained that it could not connect to the backup drive last night. Both the MacBook and the 500GB Time Capsule are at least middle-aged. Either could be failing. Console isn't showing anything suspicious on the MacBook and TechTool Pro passed it last time. The only diagnostic information I find on the Time Capsule is that the disk START status is verified. I've set syslog to 7 . I've also stopped backing up my Fusion VM image. Anything else I can do to sort out what the new failures might be from?  http://tech.kateva.org/2012/01/my-macbook-fan-was-roaring-again-time.html  https://discussions.apple.com/thread/2180886?start=0&tstart=0"
The Time Capsule error logs aren't showing anything interesting. Since the error has involved two client machines and two backup drives I'm assuming it's a problem with my Time Capsule hardware, perhaps a bug introduced with a firmware update that occurred in the past few months. I wonder if the last update has bugs with older Time Capsules and/or with older Time Capsules that use an external USB drive for backup. It's an intermittent problem, so hard to test. The only consistent feature is when it hits, "Verifying Backup" always fails. (I wonder if the bug is in the Verifying Backup, something that normally happens from time to time.)
... AirPort Utility keeps an archive of all Wi-Fi Base Station firmware updates you install, and stores them in the location below.
Mac OS X: ~/Library/Application Support/Apple/AirPort/FirmwareWindows: ~/AppData/Local/Apple/AirPort/Firmware
... Open AirPort Utility.Select your base station and choose Manual Setup, or double-click on the Wi-Fi base station icon.
Choose Base Station > Upload Firmware...From the window that appears, select a firmware version and click OK. If you select Other from the Upload Version menu, you will be prompted to manually locate the firmware update.
I have dropped my firmware back to 7.5.2; a version that had worked well for me. If that doesn't work then I'll return to 7.6.1 and do a factory reset. After that it's buying either a new Time Capsule or, perhaps better, an Airport Extreme with an external drive.
Update: I think I've got it. Verification failed 100% of the time with 7.6.x, it passed with 7.5.2.
 I'm considering adding a third, CrashPlan, for my home videos and photo libraries. I'm less concerned about the service bankruptcy security issues with those data sets than with my personal data.  I'm not entirely sure why this is so. Long ago my Dantz Retrospect tape backups worked fairly well. Over the past decade however, I've found both Windows and OS X backup solutions fail fairly frequently.
Update 3/28/12: Not a causal factor, but maybe contributing. For some reason my iMac was connecting to the standard rather than more reliable 5Ghz 802.11n band. I changed the connection setting to remove that. That cleaned up some residual issues. Although I'm out of the woods, I still wonder if my 3yo Time Capsule is ailing. Time will tell.
Update 7/17/2012: When I moved my old Core 2 Duo MacBook to Lion (mistake - don't do it!) I tried 7.6.1 again. It worked for a while, but a week ago it failed again. So I'm going back to 7.5.2.
- Time Machine: Troubleshooting backup issues: I didn't realize there was a technique for repairing a Time Capsule sparsebundle using Disk Utility, or that MAC addresses define sparsebundle names.
- Time Capsule Backup Error (-1) with Lion - MacRumors Forums: A detailed thread last updated Mar 9, 2012, no fix yet.
- Time Machine - Troubleshooting: web site dedicated to common TM problems
- Resetting an AirPort Base Station or Time Capsule FAQ
- All about Time Capsule, AirPort Extreme, and AirPort Express base station firmware updates Apple kb article includes firmware reset directions
- Time Machine Verification Issue: Apple Support Communities - somewhat similar problem in 2010 but thread continues to 2012
Somewhat related and worth knowing:
- Gordon's Tech: Time Machine backups of Aperture are not reliable?!
- Gordon's Tech: Freeing up Time Capsule space – and documentation for Time Machine and Time Capsule 12/2009 - Could Apple spare a billion to document a few things?
- Gordon's Tech: Time Capsule - using an external disk for network Time Machine backup 3/2011
- Gordon's Tech: Fixing the Time Machine / Time Capsule 10.5 "Backup volume could not be mounted" bug 1/2010. This bug was fixed in 10.5, but it seems oddly similar. It wasn't an intermittent problem though.
- Gordon's Tech: Airport Extreme: Firmware update, Time Capsule bugs, tips and fixes: 7/2009