Two HD's for Time Machine

Of course it is in fact in the data i put Macmini i7.

Thanks for the clarification on that, since a "PC" is not a Mac.

But how?

First, I assume that the HD is already formatted correctly in Mac OS Extended (Journaled) as your other back up drive is currently set up.

On the Mac.....

Open System Preferences (gear icon on the dock)

Open Time Machine

Click Select Disk....or....it might say Add or Remove Disk

Select the name of the new HD that you want to use for Time Machine backups

Click Use Disk

Now, you will see that Time Machine has been set up to back up to two different HDs....the name of the original HD that you set up previously, and the name of the new HD that you just set up.

In the example shown below, you can see that my Mac backs up to two different hard drives. One is named MBP TM and the other drive is named Backups.

In real terms I am looking for two instances of TM rather than one to alternate.

Sorry, if I wasn't clear. You will get two instances of TM backups with these drives. Both backup sets will be nearly identical at the time they are created. Again, they can not be completely identical as each backup runs individually. One will be slightly "ahead" of the other, until the other "catches" up.

But If I keep one connected and then just pull it and put the other one on once a week it wouldnt go back all week then but rather just 20m to when I pulled the other?

Sorry, but this is not how TM is designed to work effectively. What I would suggest that you do is to add a second method to your overall backup strategy.

Continue to use TM for your incremental backups using a drive that you will keep "permanently" attached to your Mac mini. For the other method, use a second drive as a complete clone of your mini's internal drive. You can then, update this clone on a weekly basis. I would recommend you consider using Carbon Copy Cloner for this task. The advantage of a clone is that if your mini's internal drive should fail, you can plug in the clone and boot up your Mac from it. You can then quickly be back "up and running."

When you are using multiple drives for Time Machine (TM), as Bob has described, both drives will alternately perform incremental backups per TM's default schedule. You cannot natively configure TM to use a different schedule for one drive over another or to change that schedule for a single drive. If that is important to you, then you will need to use a third-party app to do so.

Thanks for the clarification on that, since a "PC" is not a Mac.

Actually, it can be. The term "PC" or Personal Computer, as I am sure you are aware, came about in the late 70's, early 80's, and was synonymous with the introduction of the IBM PC, running IBM-DOS. This "tradition" continued with Microsoft's MS-DOS and today with Windows 10 on the devices that run them. However, with the appropriate hardware, a PC can run any version of OS, including Linux and macOS. So a Mac, can be a "PC."

This is one of those: "I say tomāto; you say tomäto" kind of things. 😉

But If I keep one connected and then just pull it and put the other one on once a week it wouldnt go back all week then but rather just 20m to when I pulled the other?

Not sure if I understand what you are asking.

If you disconnect the "regular" drive that you are using for Time Machine backups, and connect the drive that you want to use "weekly", then Time Machine will back up to the "weekly" drive that you have just connected.

The first time that you connect the "weekly" drive, Time Machine will back up all of the data on the Mac. After that, Time Machine will back up the changes that have occurred on your Mac since the last backup.

If you don't want to wait for a hour or so before the backup will actually occur, you can click the Time Machine "clock" icon at the top of the Mac's screen and click on Back Up Now.

The backups to the "weekly" drive are going to take longer than normal backups to your "regular" drive since Time Machine will back up all of the changes that have occurred on your Mac since the last "weekly" backup has run.

Once the backup to the drive that you will use "weekly"has completed, and you disconnect that drive and then reconnect the "regular" drive, then backups will resume normally again within an hour. If you don't want to wait for up to an hour for the next backup to run on the "regular" drive, you can use the Back Up Now option that I mentioned previously.

Time Machine generally hates any kind of anti-virus application, or other applications that claim to "optimize", "clean", "speed up", your Mac, so how well Time Machine will work for you cannot really be predicted.

Maybe if you could provide the name of the application that you are using, another user who might be running that same application with Time Machine can post back with his experience.

If you have an issue with slow backups, stuck backups, etc, and you call Apple Support, the first question that they will ask will be whether or not you are running any kind of anti-virus software on your Mac, or there utilities that claim to improve the performance of your Mac.

Can I do that?

Yes, if the "PC" that you are using is a Mac.