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Question: glacial Time Machine backup

My Time Machine is slow to back up again. For the past history, see

A week ago I tried again to back up this Mac Mini. I have a Hitachi one-terabyte external hard drive plugged in by USB 2.0. Each day for the last week I have had the same experience: Time Machine spends approximately six hours “preparing backup.” Time Machine then starts the back-up process. After another three hours Time Machine has backed up approximately 1.6 gigabytes of data. That is nowhere near enough.

Between the two computers that I back up, this Hitachi hard drive should be full. Nonetheless, my understanding is that Time Machine should delete the oldest backups in order to make room for the latest. That would be fine with me.

I admit that for the first six days I may have made an error. I turn off the Mini every night. For the first six days I shut down the Mini without my taking any specific action as to Time Machine. In contrast last night, after reading some stuff, I clicked “Skip This Backup.” I waited for Time Machine to stop. When it did, then I shut down the Mini.

Doing the “Skip This Backup” appears to have had no effect. Time Machine is still, as I write this, “preparing backup” after two hours.

Yesterday, I ran Disk Utility on the destination Hitachi. Disk Utility reported no problem with that hard drive.

What is distinctive this time is a mysterious folder that appears in Trash:

User uploaded file

This folder appeared in Trash last week after I started Time Machine. As long as the external hard drive is connected to the Mini, I cannot delete this folder.

This morning I did stop Time Machine and disconnect the external hard drive. I was then able to delete the mystery folder. After I reconnected the external hard drive and restarted Time Machine, the mystery folder reappeared in Trash. I do not know the significance of the blank folder.

Mac mini, macOS High Sierra (10.13.3), Time Machine

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Mar 20, 2018 7:17 AM in response to R_55a In response to R_55a

Update. I left the Mini on overnight. So far in 21 hours it has backed up 11.11 gigabytes. It still has 6.82 gigabytes to go.

That seems rather slow.

Mar 20, 2018 7:17 AM

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Mar 20, 2018 8:49 AM in response to R_55a In response to R_55a

If it's a hardware problem, no, also Disk Utility should run into trouble (Is the SMART status OK?). if it's a matter of files directory it can be, as Disk Repair (I have S.O.S. i my Italian Mac OS but I believe is the same) is not very efficient. In such cases I use Disk Warrior but's not cheap.

Mar 20, 2018 8:49 AM

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Mar 20, 2018 12:26 PM in response to R_55a In response to R_55a

WIthout ruling out the possibility of a bad external drive, are you using a Virtual Machine with one or more very large guest containers on your boot drive? Any change within one of these guest containers, that you do not explicitly have in the Time Machine exclusion list, would trigger Time Machine to back up the guest container — amounting to a huge Time Machine delay over USB 2.0.

Are there other concurrent, large file transfers, or downloads competing for your Mac CPU and I/O resources? Are you running any anti-virus, third-party memory management, or other ill-advised system management utilities that may be compromising the allocation of system resources to the Time Machine job?

When you monitor the Time Machine backup using Activity Monitor, is your memory pressure still green, and how low does the available memory drop? Any indication of Swap used? Are you running low on local storage?

Mar 20, 2018 12:26 PM

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Question: glacial Time Machine backup