As usual, the problem is not yours because the fault, as usual, is always someone else's. Well done. What a diagnosis.
Don't be silly, that wasn't a diagnosis, it was a request for information.
Leo is simply asking you to look in Explorer and see if you see the same thing there.
What's unreasonable about that?
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Apologies for the delay in getting back to you. The problem occurs on my computer in my office and I wasn't in it over the weekend. To reply to your question. The problem does not occur in Explorer because I do not use Explorer to sync files. The problem occurs with Dopus which I do use to sync files. If I look at the contents of my external hard drive in Explorer, all the files are always visible. My problem is that the external hard drive drops off, crashes or gets lost when I sync files using Dopus. Why? What should I do to prevent this happening?
Please try using Explorer to copy a similar amount of data and see if the problem occurs.
OK. I've now had time to use Explorer and Dopus. The external hard drive gets dropped intermittently when I sync using Dopus, but it does not get dropped with Explorer. So, it looks like this is a problem with Dopus and its interaction with Windows on my Dell Inspiron (the problem doesn't occur with the same software on the older Dell machine I have at home). All hard to explain. Any advice?
Try setting Preferences / Miscellaneous / Advanced [Filesystem]: copy_buffer_size to 60 KB. Some USB devices have problems with larger buffer sizes, and that might be causing it to disconnect all the time. (If it doesn't help, reset it back, since it can affect copy performance.)
Opus only calls high-level APIs like CreateFile, ReadFile and WriteFile, so if the drive keeps disconnecting it's going to be a fault in the drive or USB device/driver it's connected through. There's nothing in Opus that would disconnect a drive, short of the Eject command being run. But we have seen USB devices that don't work properly with larger buffer sizes (as well as ones that don't work with non-buffered I/O, but that's off by default and you'd know if you had turned it on), where the devices were apparently only ever tested against the buffer sizes Explorer uses.
I would also check in Event Viewer under Windows Logs > Application and Windows Logs > System. There may be Error or Warning events in there coinciding with the times of the disconnections, which could give more detail on when or why the disk is disconnecting.
If you can capture one of the disconnects with Process Monitor we could look at that to see if it reveals something as well. A similar log of what Explorer does with the drive could also be useful for comparison (that would tell us the buffer size Explorer is using, for example; it varies by device and connection type).