At what stage are you seeing this?
Is this external drive NTFS formatted? It shouldn't make any difference because all you are trying to do is read the drive and Macs can read NTFS, but not write to. However, maybe something in the copying process is trying to wrtite information to the drive, possibly Spotlight seeing this as a drive that needs indexing (guessing here)?
I'm seeing this after 40GB has copied across, out of around 100GB. You're right, the drive is NTFS formatted but, according to all the Apple tutorials I have read, this shouldn't be a problem in itself. There may be one or two erroneous files within the iTunes folder I'm trying to copy across - it is old, in as much as I started using iTunes pretty much when it was first released - so maybe I need to just copy across what I think I need. But that worries me, being not that tech savvy and, of course, I don't want to compromise my next sync (and first with the Mac) to my iPhone..... thank you very much for your reply, whatever the outcome....
My feeling is there isn't much you can do about it since standard things such as changing file permissions won't work if you can't write to the drive. You could try formatting the drive to FAT so both the PC and Mac can use it (as long as you don't have any huge files). Alternatively, if you have a smaller flash drive formatted FAT you can move the library in groups of folders as long as you make sure it all goes back together properly again.
Here's my standard post on moving libraries:
If you use iTunes' default preferences settings: Copy the entire iTunes folder (and in doing so all its subfolders and files) intact to the other drive. Open iTunes and immediately hold down the option (alt) key (shift on Windows) so you get a prompt to select a library, then guide it to the 'iTunes Library.itl' file in the moved iTunes folder.
If you put the copied iTunes folder in the default location of Macintosh HD > Users > *User Name* > Music then you don't even need to start with the option key held down, iTunes will automatically look for it there. (Make sure there isn't anything already in the iTunes folder there that you want to keep since you will be replacing it with the one you are moving.)
Longer answers if for any reason you do not think all library files are inside your iTunes folder (changed preferences settings at some point):
iTunes: How to move [or copy] your music [library] to a new computer [or another drive] - http://support.apple.com/kb/HT4527 - a somewhat bewildering and not always easily understandable set of options.
For the record there's this reference for iTunes 11 but it really doesn't strike me as having the specifics you need. iTunes 11 for Mac: Move your library to another computer - http://support.apple.com/kb/PH12168
2011 ilounge article: Transferring your iTunes library - http://www.ilounge.com/index.php/articles/comments/moving-your-itunes-library-to -a-new-hard-drive/ - An article with good background information (similar to the links above), particularly if you are not sure your media files are all grouped for quick answer above. Unfortunately under the single term "transferring library" it describes two very different moves; relocating just your media (not really transferring your library and not recommended unless you really need this configuration), and relocating media+database (really your whole "library"). Make sure you understand the difference before electing to move only media.
Windows users see tip at: https://discussions.apple.com/message/18879381
Thank you Limnos.
If you're interested - and for anyone else who experiences this problem - I found a fix. Firstly, by copying the folders within the iTunes main folder indivudally across to my Mac, rather than in one fell swoop, I figured I could at least discover which folder was giving me the problem. It transpired that the problem was within the main iTunes Music folder. It simply stopped copying at around 40% in but, crucially, left the stuff already copied across on my Mac. And as it copied alphabetically, I was able to see on the Mac (via Finder, naturally) where it had stopped and then, by a process of elimination (copying and then seeing when it denied me that ability) around that point in the alphabet, I narrowed it down to one album folder, within an artist folder. I have no idea why that particular album was not copying across - it was a De La Soul freebie they had posted up on their website - but I simply excluded that album from the copying process, carried on copying everything bar that from the PC to the Mac and hey presto. N.B. I then emailed myself the tracks within the album that wouldn't copy and re-ingested them into iTunes, although I'm sure that's not the techie way of doing things. But it worked....
Thanks again. Your generosity has not gone unnoticed.