Backup with Time Machine on 2 different Hard Drive

Question

Hello,
I'd like to know if I can backup *all my data on 2 different external hard drives*. The aim is to a double backup in case of big issue.
I'd like to process on this way:
- only one of my backup HD at home and the other is at the office
- when I plug one of the backup HD, its data is updated according to the previous backup with this HD (not with the other one)
- if I backup my MBPro just before leaving the office (on the office's HD) and I backup my MBPro just after arriving at home (on the home's HD), my 2 HD should have the state and exactly the same backuped data.
Is it possible ? Otherwise, is there another way to proceed ?
Thanks for all,
Malphas

Answer 1

malphas,
*Using Time Machine with Multiple Destination Hard Disks*
Yes, this is possible, as long as you understand how it works. First, Time Machine is only designed to work automatically with one backup disk. If you are going to employ multiple Time Machine disk with a single Mac, then you are going to have to manually select the new drive each time it is attached using the Time Machine Preferences “Change Disk…” button.
Secondly, Time Machine *+does not+* ordinarily perform file-by-file comparisons to determine what has changed and thus determine what needs to be backed up. Rather, Time Machine relies on FSEvents notifications. This is a log that the system uses to keep track of changes to directories. Rather than scan tens of thousands of files for changes each time, Time Machine simply looks at this log and narrows its’ scan to only the directories that have experienced changes since the last backup.
Every event that FSEvents records has its’ own ID which includes a time stamp. At the end of every backup, Time Machine stores the last event ID that it processes. When the next backup is initiated, Time Machine looks at this stored ID and determines that it only needs to backup events that have occurred after the time stamp on this last event ID.
Naturally, for hourly backups, Time Machine does not have to go back very far to find the last known event ID. However, if a Time Machine disk is only being attach for backups every few days, a week, or more, and Time Machine has to go back too far to find the last event ID, then it will give up and simply go into “deep traversal” and do the file-by-file scan on its’ own. There are simply too many events logged by the system for Time Machine to bother looking for the last known event ID. Consequently, expect lengthy “Preparing Backup…” sessions anytime you attach a backup disk infrequently.
Nevertheless, using Time Machine with multiple destination disk will work. If the two drives are routinely backed up then they will always remain in relative sync.
Finally, remember to reselect your normal Time Machine disk in the Preferences when the backup to the secondary disk is complete.
Hope this helped clarifies things a bit for you.
Cheers!