The hard drive is formatted as a Mac HFS+ volume. Windows can’t read it without some third party software
MacDrive www.mediafour.com/products/macdrive
Paragon HFS+ for Windows www.paragon-software.com/home/hfs-windows/
HSF Explorer (open source, read only) www.catacombae.org/hfsx.html
Although both Windows and Mac can read and write FAT32, if you have been using it on a Mac don’t change it to FAT32, you will lose all of your data and you will no longer be able to use it for TimeMachine, boot a Mac with it, or write large files over 4GB to the drive.
If you are willing to erase the drive (and lose all the data) you could partition the drive with the Mac DiskUtility, leaving the first partition HFS+ for Mac bootability and TimeMachine, and formatting the second partition to MS-DOS (FAT32) so that you can exchange files from Mac to PC and back on it.