Time Machine backing up to USB and NAS, but it is alternating every other hour instead of backing up to both every hour

I do not believe that this Time Machine behavior is user configurable. It backs up once per hour, and alternates through all the available backup devices in turn. You get a backup every hour, but each device's incremental backups have intervals of n x 1 hour where n = the number of devices being rotated through for backups.

You can instruct Time Machine to "backup now" manually in the menu pulldown item when a backup finishes on device #1 and that will mean a backup starts immediately on backup device #2,

Note that even if you do this, the backups on the two devices will not be identical. And the offsets between them and their contents will depend on how long that last backup took on the other device. Usually incremental backups take a minute or two from what I have seen, but sometimes after a major installation or large file modifications, the backup can take tens of minutes or even more than an hour, and the backup speeds will be different for a locally connected SSD versus a network storage device. So you would always need to be figuring out the most recent backup when you have multiple devices storing backups, even if you could program the behavior you wanted.

There is no means to update all backup targets for each Time Machine run.

Time Machine backups rotate through the available targets. Always has.

Each backup target gets the changes since the last backup, so all backups are consistent, and current to the last backup to that target.

Time Machine also uses local snapshots, particularly when the backups are not accessible.

Back up your Mac with Time Machine - Apple Support

Use Time Machine on your Mac to back up to multiple disks - Apple Support

About Time Machine local snapshots - Apple Support

Slightly dated, but useful: http://oldtoad.net/pondini.org/TM/Home.html

What does, however, is backs up to the first device, then an hour later the second device, then an hour after that back to the first device. So it is backing up to the same device every two hours instead of every hour.

That is normal Time Machine behaviour.

How can I get it back up to both devices every hour?

You can't do that. However, TM automatically creates "Local Snapshots" constantly. They are fully restorable complete system backups.

CJonesCA wrote:

Lame. If I have a crash and don’t know what target device it used last, because why would I be paying attention to that, I have to figure out which has the latest. Otherwise, I could restore to a copy that is almost two hours old instead of a more recent one. Kind of dumb.

If I need a recovery from the last hour of active use, yes, I would want to check which of the available archives was most recent.

You’d still need to verify the latest backups with sequential backups, though presumably with a somewhat smaller window for changes. And with enough targets or with sufficiently slow hardware or connections, sequential backups would be running continuously.

Time Machine is intended to be simple, effective, reliable, integrated with migrations and network restore, and easy to manage. Not tuned and tailored. If you need continuous backups, or need detailed controls over backups, then Time Machine is not your choice.

No way to change the behavior in the com.apple.backupd-helper.plist or anything?

There is no means to update all backup targets for each Time Machine run.

You’d still need to verify the latest backups with sequential backups, though presumably with a somewhat smaller window for changes. And with enough targets or with sufficiently slow hardware or connections, sequential backups would be running continuously.

If you are restoring a full backup, each are datetime-stamped.

If you "Enter Time Machine," you don't pick the destination--it appears a single backup, ordered from most recent back.

Does yours not appear that way?

Barney-15E wrote:

You’d still need to verify the latest backups with sequential backups, though presumably with a somewhat smaller window for changes. And with enough targets or with sufficiently slow hardware or connections, sequential backups would be running continuously.

If you are restoring a full backup, each are datetime-stamped.

If you "Enter Time Machine," you don't pick the destination--it appears a single backup, ordered from most recent back.

Does yours not appear that way?

I’m referring to which Time Machine backup archive gets restored of those that were being maintained. The source of the restoration, not the destination of the restoration. Some of the Mac systems I administer can have two to as many as four different Time Machine targets all active, some local and some on NAS and—for laptops shuffled between locations—sometimes on completely different networks.

With the proposal to have continuous sequential backups (e.g. so all four targets get a backup each hour), those backups still require a chunk of time to complete, which means each of those sequential backups is different. Which means the same issue as the every-hour backups, albeit with the windows of differences sized to the duration of each of the sequential backups.

I really don’t want to think about a multicast backup scheme (RAID-like writing to multiple backup target archives entirely in parallel) (this to further reduce the backup window) either, as that’s just asking for trouble when one of the backup targets glitches. And one of the targets will glitch, whether network instability, or disk hardware error, or a mains power wobble, or USB glitch…

Barney-15E wrote:

I’m referring to which Time Machine backup archive gets restored of those that were being maintained.

So was I. I'm not sure why you think it any different.

I’m referencing picking restoration source. If I have four separate backups, they’ll each have slightly different contents based how much of the last few hours were captured on each, and—when restoring—I need to specific which of the four backup archives I wish to use as the source of the restoration. I can select any of the four as the source of the restoration, and the contents of the backup will restored. I’ll obviously usually want to select the backup source with the newest bits, but not always.

I’m referring to which Time Machine backup archive gets restored of those that were being maintained.

So was I. I'm not sure why you think it any different.

They all have timestamps. I don't know how that is difficult.