In order to be able to use your external harddisc both with windows and mac, you have to format your harddisc to the FAT32 Filesystem
Additional Info: (Not relevant for the procedure) Windows 7 / Vista does not support formatting FAT32 natively, but you can still do it with the command line tool.
Attention! Formatting means, that you will lose all of your data on the external disc! So don’t forget to backup your files!
Option 1, Using the command line:
1) Open a command prompt window by going to Start, then Run and type in CMD.
2) Now type in:
format /FS:FAT32 X
Where (X) stands for the driver Letter, for my case, i’ve used the command
(format /FS:FAT32 H)
Option 2, Using the command line tool fat32format
1) Download the file fat32format
2) Extract the file into C:\
3) Open a command prompt window by going to Start, then Run and type in CMD.4)
4) type in the command “cd ..” till you get only “C:\>”
fat32format.exe -c16 H
(where H is your drive Letter and -c16 the sectors/cluster option, fore more info on that just type in fat32format.exe without any options