Finder error message when copying many files

Hi...

I was copying a huge amount of small midi files (over 64,000, but probably only around 400mb) from one external drive to another. Towards the end copying stopped because I got a finder error on one of the files.
I know this stuff would happen sometimes when copying large amounts of data on my pre OSX systems.

Generally when that happened, I would copy smaller chunks. This also worked for me in this case and was able to eventually copy everything without error.

So my question is this: Why would I get an error in the first place? Is there usually a potential for errors when copying large amounts of data at once?
And is this normal behaviour?

Thanks in advance!