progress bar?

Just how many pictures are in each folder (and are they jpegs or RAW or some other large file format)?

I'm going to offer a guess: the folders appear to have a progress bar > so, if the transfer from the external is not complete, it may show that progress bar and the "x" would indicate you can't open the folder until all has been downloaded.

Please answer babowa's question and then we can get more information on how to help you. I'd start by resetting the NVRAM with the External Hard Disk's disconnected and then retrying. Also reset the SMC with all peripherals disconnected 2-3 times and restart in Safe Mode to clear any corrupt caches. Instructions for each of these are in:

PRAM and NVRAM Reset

SMC Reset

Use safe mode to isolate issues with your Mac - Apple Support

Frankly, I have no idea except that you "overworked" the computer. Try to import them one folder at a time (or even by subfolder) because RAW pics are usually at least 10 MB for one. If you are not sure, I would delete all of them (please also one folder at a time) and then try it again, this time one folder or subfolder at a time.

suze2019 wrote:

Each folder has several hundred photos in it, in jpg and raw format, and some have subfolders.

You system is choking on the amount of copying you're throwing at it! Take babowa's advice.

Transferring from one external to another is much slower than transferring from internal to an external or vice-versa. I agree with the others and also suggest that, you only copy one or two folders at a time and wait until those have completed before coping one or two more.

Please explain in more detail exactly how you did what you did.