How to create mirror backup of my Time Machine external hard drive

It's not a good idea to back up a backup. Instead, back up directly to both external drives.

Time Machine will do something like that automatically. Just connect the second drive and ADD it to Time Machine. It will automatically create a second, independent, Time Machine Backup, NOT based on the first Backup. From that point, it will alternate updating the Backup set on each drive. Hour 1 will update the first Drive, hour 2 will update the second drive, Hour 3 will update the first drive again, ...

As Linc Davis suggests,there is a strong argument to be made for a Backup using a completely different backup method. Then if there is a weakness with the first backup method (and your data falls into its weakness) you have another view of your data, and more hope of saving it.

This! http://bombich.com/features

It will alert you to corrupt files and try to replace them. It can do a file checksum to insure integrity.

And you can always have a spare volume should you need one, including your system, and a way to keep or move a volume off-line, secured or to a client or another workstation. As well as archiving file changes.

TimeMachine is fine, as far as it goes, but "never put all your eggs in one basket" and it can develop problems with its catalogue.

If you want to be safely paranoid then you should always have a minimum of two different backup sets, last week and last month, as well as daily (M-T-W for instance as rotating set).

At work we didn't call it a mirror, but we would dump a disk to tape every night, two, because tape could not be trusted, and so one tape would go into a locked cabinet. And clean those tape heads! was a must of course, as well as dust free (no smoking) as tiniest particles could kill a backup operation. So it had air scrubbers in the air filtering system to get some micron ? level stuff from ever getting into the air. (If I had my way, even smoke on clothes would be banned! serious)

Anyway, CCC is a great product, been using for 13 years, there are others too. How sad so many backup suites over the last 25 years have come and gone....

Drives fail. Often in the first six months, and after that, they may last 5-6 years. Or not. A weak (not just bad) sector block is usually the best indication. Having spare drives on hand should be a must, and rotate primary drives to backup or archive every two years maybe after giving them a long erase - to ease your fears and hopefully find the worst possible error.

RAID6 is useful for holding large video catalogues and projects - and then mirror your RAID6 to another RAID6.

SoftRAID 5 http://www.softraid.com will scan drives in the background for weak blocks, monitor I/O better for read/write errors (without impacting performance is important) and can be well worth its $140 ish price (worth it in tech support alone, too if ever you need it)

A couple programs will write the SMART status to the system log and show the numbers of used and remaining spare blocks. That is something a large study of disk storage revealed. And in today's world, storage needs and the need to "change the light blubs" on a basis before one fails is a must in massive data cloud and big data mining.

Apple's Mac Pro and the RAID card is not "hot swap" capable, though the motherboard in the Classic Mac Pro actually could as designed by Intel. And hot swapping is a handy feature - create a 3 drive mirror array, pull one, you still have two and let SoftRAID do a better rebuild in the background adding a new 3rd mirror drive.

Two drive mirrors are weak and not all that great, in my estimation.

TimeMachine is there to get people to at least have a automatic ish do it and forget it, hopefully, versus none at all. A good beginning. It also turns 8 this September and has slowly, gradually, hopefully, matured. I might buy Disk Warrior 5 if only to check the backup catalogue for integrity though at $99 a waste for most and better spent on another backup drive or something. But you never want to be in need of Data Rescue 4, another $99 product.