Backing Up Images on Portable Hard Drive - Cyclic Redundancy Errors

[Alis]

I had a back up drive for some of my image files (WD passport portable harddrive) and now that I am trying to access them it gives me a lot of "cyclic redundancy error". Anyone has any idea what causes it and how to recover those files?

[Colin Southern]

Whenever data is written to a drive a small additional "checksum" number is added to each file, with the actual number being calculated based on the contents of the file.

When the data is subsequently read, the CRC number ("checksum") is recalculated, and should come to the same value - if it doesn't then it mens that the contents of the file have changed (due to a mis-read, corruption, or other malfunction) - a CRC failure is another way of saying "there are errors in this file", and standard practice is for the operating system not to give it to you.

To answer your question "what causes them?", the simple answer is - usually - impending drive failure. You may get one or two on the odd occasion if the power to the drive has been glitched (even though it's not supposed to be able to happen), but other than that, it's a MAJOR warning sign that all is not well with the drive, and you'd be well advised to get any importnat stuff off it while you can.

To answer your question "recovering those files", the answer isn't so cut and dried - personally my main weapon is a program called SpinRite by Steve Gibson from Gibson Research (www.grc.com), but getting it to run over a USB connection can be an issue (for 2.5 inch drives we often connect them directly to a PC via an adaptor) - there are other utilities out there that may help (or they may make things worse), but the unfortunate reality is that you're now entering the realm of DR (Data Recovery) where firms like DriveSavers (www.drivesavers.com) have a venerable reputation for success, but they charge like a wounded bull ... and it's usually uneconomic. This is the reason why people need multiple backups for data, unfortunately.

One VERY "quick and dirty" trick that I've used in the past with occasional success is to put the drive in a fridge for several hours - and then (with the drive wrapped in a towel to absorb condensation), try to copy the files as fast as you can - it works about 30% of the time, but it's obviously a risky thing to do so I'd suggest only considering it as a last resort - AND - if there's nothing else on the drive that you want to keep.

Cheers,

Colin - pbase.com/cjsouthern