Read/write to a Mac external HDD on PC

[Kain]

My uncle has a Mac and an external HDD. He reads and writes stuff on/to it using his Mac. I have a Windows 10 PC and he brought over his external HDD. I was able to read and write to it as well. I thought Mac and Windows used different file system formats that are not compatible. How come I was able to read and write to it?

 

[Samuraicow]

Your uncle's external hard drive is probably formatted to exFAT, if it was formatted to HFS+ you would have probably needed to use a software to access the content...

http://guides.macrumors.com/File_systems