USB Drive read/write 100%... very slow write

Run a benchmark test
Suggest to use Crystal Disk Mark . It is a portable, don't need to install. Extract and run.
Post a image of the results

Well, if the fact that there is little space left is the key factor, it is odd that I don't have the same issue with the identical drive I mentioned, which has the same amount of free space (more or less). In addition, the copying starts off very quickly and then suddenly slows down: seems unlikely it suddenly hits a 'critical' free space level. The free space I mention takes into account all the space needed by the drive itself of course - I expect to get about 5500MBs on a 6GB drive, maybe a bit less.

The other thing that makes me wonder whether the problem is with the disc itself is that the problem is sometimes not apparent at all, especially (as I said) if the size of the folder being copied is relatively small. Yesterday, instead of copying one folder (containing 30 subfolders) of 30GBs, I broke the operation into smaller bits, as it were, and dragged over each individual subfolder to the drive in question: that's 30 folders of about 1GB each. The speed with which each one copied over was amazing, just a few seconds for each one - the speed showed about 200MB/s, which was about right by the clock.

It seems that the sudden, very drastic slowing occurs after a certain amount of data is copied in one go. If you do it in smaller pieces, the files copy over before the 'buffer limit' (if that is what it is) is reached.

Not sure if that makes any sense technically, but it is certainly what happens in practice - I copied all my files in record time by doing it is increments like this, just a couple of minutes, if that, (as opposed to the "120 days" estimated yesterday, at 150kb/s, or whatever). Also, the read-write % stays reasonably low and goes back to 0% when the copy is finished, rather than sticking at 100% until I reboot - even when no files are being copied.

(I have also freed up a bit of space.. there's about 200MBs free on the drive at the moment: I use up a lot of storage space with what I do (satellite earth terrain images) - I have at least 15 drives ranging from 2 to 8TBs: I try to maximise the use of each one as far as possible for $$$ reasons!).

I will try a benchmark at some stage soon too, but it'd be good to know why I can copy up to around 5-6GBs at a time (it seems) with no issue: if I copy more than that in one operation, the drive seems to hit a wall and doesn't free itself until I reboot. It's actually not the first drive I have had which shows exactly this odd (?) behaviour.

Thanks.

Nothing is wrong as such, the same thing happens when using File Explorer to copy files to a USB thumb/flash drive.The difference is the HDD has a buffer(cache) which accounts for the drive working after cancelling.

It is particularly noticeable with large numbers of small files.

As a simple experiment just ZIP your small files so you are just copying 1 large file, then you will find speeds are much better.

I am no expert but the fluctuations seem to be something to do with a combination of the way the USB interface works and how the disc filing system works.

The first M Bytes of a file are transferred to a buffer (RAM) and then from the buffer to the disk. That is why you see very fast transfers and then it slow down.
Transferring a big file is completely different of transferring many files in sub folders.
Did you run benchmark test?

Make sure Write Caching is enabled. I think Windows standard policy is set for Quick Removal instead of speed. Double click on drive -> Propierties -> hardware -> properties -> change settings -> policies (or directives) (taken from Spanish).