Hi Murry,
It can indeed be a difficult task to transfer the cache data. I have a couple of considerations for you:
1) Have you thought to migrate to the compact cache which was introduced in ArcGIS 10? You can do this by using the GP tool Server toolbox > Caching > Convert Map Server Cache Storage Server (or just by caching the map service to the compact cache). Compact cache files are much larger (and there will be fewer of them than those .png files you have for sure) thus it goes *much* faster to transfer them.
2) Another thing I have seen a couple of times - it took a bit longer time to copy massive data in Windows when files have longer names. It may be not directly relevant to your case, but still worth to check: http://support.microsoft.com/kb/130694.
3) There is a KB article: http://support.esri.com/en/knowledgebase/techarticles/detail/33234 where you can read on ways to copy the cache data.
4) Maybe you can archive the cache data and then copy the archive files? This should definitely boost the performance. Then just exclude the ArcGIS Server cache folder in your backup software, so it will not choke when copying cache tiles.