Trying to copy across my iTunes library to my newly wiped MacBook but all songs greyed out and iTunes folder now 0bytes...

I use an older operating system version and do not fully understand the view you have. It doesn't look like a standard iTunes folders structure to me:

What are the iTunes library files? - http://support.apple.com/kb/HT1660

More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management

What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html

Where are my iTunes files located? - http://support.apple.com/kb/ht1391

iTunes 9 [and later]: Understanding iTunes Media Organization - http://support.apple.com/kb/ht3847

Image of folder structure and explanation of different iTunes versions (turingtest2 post) - https://discussions.apple.com/thread/2741246?answerId=13025536022#13025536022 and https://discussions.apple.com/thread/3700206?answerId=17457605022#17457605022

If you do have a complete library set as per all the links I provided above, here's how you move it: 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.)