I have a iMac desktop with a SSD (not Fusion) and want to do a Time Machine backup on an external (regular) HDD.

Should Disk Utility be used to erase/format the HDD in APFS or the "old school" JOURNALED-EXTENDED?

You need the old-school Journaled-Extended. See How to format a disk for Time Machine

Thanks.

Should have checked earlier (but . . . if anyone is still awake at 10:00 pm on the West Coast . . . )


Will DU continue to erase and format the HHD even if the computer is put to sleep . . . or will the process be interrupted until the computer is running full again (and then pick up erasing where it left off)?

P.S. I would like to run the erase overnight - since 2TB takes hours - but not have the computer at full throttle while doing so, if possible.

Two hours? That's strange. When I erased my 2 TB HDD (connected via Thunderbolt 2), it took only a few minutes. If you are using one of the secure erase options, that could explain the difference. NOTE: USB 3 is nominally half as fast as Thunderbolt 2, but in practical terms it makes little difference.

AFAIK, putting the computer to sleep will pause the operation and resume when woken, so it's best to leave the computer running (although you could put the display to sleep).

Two hours? If only.

2TB takes at least 6-7 hours with double-pass erase using USB3 on Western Digital's "My Passport."

Putting the DISPLAY to sleep works for me. I don't mind the computer running so much as I do having the screen burn away through the night.

Just wondering: With the DISPLAY ASLEEP, should I also set the SCREENSAVER to "NEVER" in the Preferences or (by default) will it not try to kick in when the display is asleep?

My screensaver preference is set to NEVER because I don't use a screensaver, and, as I understand it, it's not necessary unless you have an old (very old) CRT display. So, I can't answer that question.

Why do you want a double-pass erase? If you think that someone will steal your hard drive, I can understand that, but if you are the only one using it, that procedure, to my mind, is overkill.

I would do a SINGLE (zero) pass only, but that is not an option in DU as far as I can tell.

The options showing for me are (from fastest to slowest) are:

No passes
2 passes
3 passes
7 passes

Unless there is a compelling reason to use anything other than "no passes", you're making extra effort for yourself. As the drive fills with new data from Time Machine backups, any lingering data will be overwritten.

The Display Sleep setting overrides Screensaver. Even if screensaver runs, the screen remains blank.

As for your double pass erase, the only reasons I can think of to perform a secure erase like that are:

1. You have reason to suspect the drive came from the factory with pre-installed malware (It has been known to happen)
2. You are passing the drive along to unknown parties and you have had sensitive data on the drive
3. You have a reason to suspect the drive might be failing and want to force any questionable data sectors to be re-mapped to spare sectors.
4. Your computer will be or has been used in highly sensitive data environments such as the financial industry or DoD. However, in that case you should use the 7 pass erase which can take days rather than hours to complete.

In which case you probably shouldn't oughta be using the drive for Time Machine.

Nope. Brand new 6 months ago. It has just been used with the "old" iMac and I wanted to wipe it clean to set-up with TM for the new iMac.

All erased and the first FULL backup has been done. No glitches as far as I can tell.

If used as intended, there will only be one FULL backup in the entire TM backup data set. From that point on only changed files are written to the backup data set.

Re: " . . . first FULL backup . . . "

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Yes, I realize that, but I see by the way I wrote how you may have thought I didn't.




P.S. The double-pass erase on 2TB with DU took over 8 hours. (I went to bed at that point and the job was done when I awoke in the morning.) The full backup of 380GB with TM took 4 hours. I used the USB3 on the EXHD with an adapter plugged into USB-C on the back of the iMac - if that makes any difference regarding speed.

(WARNING: this may be more than you want to know but here it goes anyway.)

It does make a significant difference but perhaps not quite in the way you envision. USB 3 is a data communications protocol while USB C is a type of connector and can be used for either USB or Thunderbolt 3 data communications. On top of that USB 3 covers three different protocols four or five marketing names and speee ranges from 5 Gbps to 20 Gbps. This Wikipedia Article does a good job of detangling the relationship between USB protocols and connectors.

According to Apple's specs the iMac has

• 4 Type A USB 3.0 ports (Up to 5 Gbps)
• 2 USB C ports capable of
  • Thunderbolt 3 (40 Gbps)
  • USB 3.1 (Up to 10 Gbps)
  • Display Port (with adaptor)
  • Thunderbolt 2, HDMI, DVI, and VGA (with appropriate adaptors

So if your eternal drive is connected via one of the USB type A connectors the best speed you can get is 5 Gbps, but if you are plugged in through a USB C connector you could get up to 10 Gbps. But since the speed is limited by the slowest device the USB port on the disk drive could be the limiting factor and I have no idea if it is USB 2.0, 3.0, 3.1, or 3.1 Gen 2 (a.k.a. 3.2).

To be perfect honest USB 3.0 (5 Gbps) is generally fast enough for Time Machine backups. The only time you are likely to be aware of the speed is during an initial backup or a full system restore.

This is what I used: " . . . the USB3 on the EXHD with an adapter plugged into USB-C on the back of the iMac . . . "

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So . . . it looks like I used a TYPE A (on the EXHD) plus the USB-C connector (adapter) into the Thunderbolt 3 port in the back of the iMAC.

I have done subsequent backups since yesterday and 7-8 GBs takes just a minute or two to run its course.

I seriously doubt that you are getting any detectable boost in throughput by going through the USB C port. Your EXHD is the choke point.

OK. In that case, I will not use the adapter and simply plug in the USB cable from the EXHD directly to the back of the iMac.