Hi, if the hard drive is formatted for Mac (HFS+) it wont be recognized natively by a windows computer, in order to make it compatible with both you can format it on FAT32 (4GB limitation per file) or exFat, this will delete your files so do a backup first. There is also a program you can use to read HFS+ partitions on a windows PC, check the link below.
http://www.catacombae.org/hfsx.html
Edit: Too late 
