The image quality and the type of storage media being used are NOT related.
You should concentrate on WHAT you are storing, not HOW you are storing them.
If you are able to see anything on the PC, there is no problem in terms of being able to access the files - you just need to ensure you are transferring the correct full resolution files to the hard disk.
Perhaps you are transferring RAW camera images? It is possible that your PC software is only showing you the embedded JPEG contained within the RAW file, whereas the MACs software would be interpreting the full RAW image data. The solution would therefore be use better PC RAW image software, such as Adobe Lightroom.