Why does copying files start fast then slow down so dramatically?

18 months on (almost) and the problem is no better.

I am currently copying (or trying to, at least!) a large number of files (100GB+) from one USB3 external HDD to another. All files are exactly the same size, 11MB. I select all the files and copy, either right click and 'paste' or with Ctrl+V. I get about 48MB/s to start with but within minutes that falls. Several minutes later I am at 256kb/sec and the estimated completion time is over two days. The speed will come up again after a minute, but eventaully I never get much faster than 7MB/s or thereabouts as a maximum; more often under 1MB/s. What (expletives resisted) is going on!?

If I reboot, I get the 48MB/s back for a few minutes, but it soon slows down again. Both drives (one a Lacie 5TB, the other a WD 4TB) are enabled for 'Better Performance' and have write caches enabled. Is there any more I can do? I am sure the problem is Windows, not the drives, because I get this on all the external drives I have, though to varying degrees perhaps.

Copying via script (XXCopy) also slows down... I already use TeraCopy for moving/copying files... No AV running...

CheckDisk showed zero errors on the drives. Windows Defrag showed 0% defrag, which is very unlikely. Maybe the innate defragger can't deal with drives of this size?

Anything else that might help?

Thank you.

That's about right!

The reason why the problem seems a lot worse when I use a particular drive, a Lacie 5TB (though as I say, none of them is immune, to say the least) is that it is probably on the blink... it seems to lock up Windows Explorer every so often and prevent the PC from shutting down: the disc light just keeps blinking very quickly. It is more likely to slam on the read/write brakes than my other external drives... As it is still under warranty (just!), I am returning it for a replacement, since I don't seem to be able to get a refund after almost 2 years (and have ordered another (4 TB WD) drive to replace it meanwhile: expensive business this!).

Nevertheless, all my drives suffer from the same malaise to a certain extent (courtesy of Windows 7 I imagine?), so any suggestions for improvements are still very welcome.

(Is there any way to get the files to copy in 'DOS', on reboot, before Windows boots up. Or maybe this possibility was lost long before Windows 7? Or perhaps that wouldn't help anyway??).

Thought I'd try the copying via my Windows 10 laptop, but there's the same problem. The speed is all over the place.. 4-5 days estimated (at worst) to copy 2TBs. 18 hours, at best (welcome to 'ultra fast' USB-C!! How long should copying 2TBs take in theory, according to the USB specs?).

I guess that, yes, that is Windows... nothing much to be done about it.

One thing I have noticed over the past several months of trying to copy large amounts of data (mostly via USB, HDD to HDD): if I drag and drop one single folder, no matter how large it is, from the source to the destination, the copy speed almost never slows down. It can run for an hour or more at a reasonable speed. However if you drag and drop (or copy and paste via the keyboard) multiple folders (including folders containing subfolders), then the copy speed invariably drops to a crawl. Similarly with a batch script...

So in desperation to get stuff backed up this week I have spent hours dragging individual folders across from one screen to another - an annoying waste of time, but at least the copying goes ahead 'full speed'. (I have also, for the largest folders, containing hundreds of subfolders, written batch scripts which copy each folder separately - the script may have hundreds of lines in it. That also runs without slowing down, usually, but can take a long time to set up).

Fortunately, I have finished 95% of the backup now - some 5TB of data - and hopefully won't need to do this again for a long time to come (fingers crossed), but why should what I have written above be so? Any ideas? The files being copied are all of identical type and size (11MBs).