Sudden drop in Macrium Reflect backup speeds under USB boot rescue environment

This one is likely specific to my machine, but hey.

 

Problem:  Macrium Reflect backup times in the WinPE boot rescue environment (i.e. booted from a Macrium USB stick) suddenly change from acceptable (~500-700 M/s, 1-6 hours) to ridiculously slow (< 40 M/s, 100+ hours).

Macrium starts well, but "hesitate" about 3-6 seconds after starting a backup (no updates on screen), after which read speed is updated to a very slow number.  Additional hesitations may occur as Macrium struggles to read the source disk.

Speeds under Windows proper are fine, and this will occur when writing to multiple backup targets  (i.e. to NAS, to another internal HDD/SDD, to an external USB HDD, etc.)  There are no indications of problems with either the source disk or the target disk.

 

Possible solution:  Try swapping your source drive to a different SATA port.

This is a nasty one, as no one expects a SATA port to misbehave.  Because they never do.

In my case, I thought it was the SSD, so I made a clone, and updated the firmware on both.  But swapping in the updated clone SSD did not fix the issue.

So: it's not the source; it's not the target; it's not the software.  What's left?

Fortunately, after running more experiments, I saw that that both drives (original and clone) behaved well in my SATA HDD dock, but behaved badly when plugged in as my boot drive port.  And plugging my original boot drive into an unused SATA port finally fixed the issue.

 

I do not know why the SATA port that has worked well for years suddenly decided to misbehave.

I also have no idea why this only occurred under the WinPE environment.  My guess is that the WinPE drivers are not as sophisticated as the drivers for full Windows, and struggled to deal with the failing SATA port.

And yes, I could back up with Windows live and running.  But I come from an era where that is a big no-no, and I still do not trust it.  I only back up after booting from alternative media.