backup of backup or alternating backups?

[Default User <hunguponcontent@gmail.com>]

Hi!

On a thread at another mailing list, someone mentioned that they, each
day, alternate doing backups between two external usb drives. That got
me to thinking (which is always dangerous) . . . 

I have a full backup on usb external drive A, "refreshed" daily using 
rsnapshot. Then, every day, I use rsync to make usb external drive B an
"exact" copy of usb external drive A. It seemed to be a good idea,
since if drive A fails, I can immediately plug in drive B to replace
it, with no down time, and nothing lost. 

But of course, any errors on drive A propagate daily to drive B. 

So, is there a consensus on which would be better: 
1) continue to "mirror" drive A to drive B?
or,
2) alternate backups daily between drives A and B?