Unable to edit\Write to external drives

When attaching a hard drive to my imac I am can copy files from it to my local disk, but am unable to edit the drive\files while they are on the external drive.

I.e. I want to remove a file or I want to save a file, or create a new folder on the external drive it wont let me. I am showing as a administrator but not sure what other settings would be stopping me from doing this.

If I attach it to my work Laptop (windows) it works fine ... any thoughts?

Thanks.

It is likely that the drive is formatted as NTFS which is only partially supported in OS X - i.e. you can only read the disk. Your options include formatting it with FAT which is Windows and OS X friendly but formatting will erase all the data on the disk or you can install something like ntfs-3g to get read/write support for NTFS drives in OS X.

Note that if you go the FAT route, FAT32 can only handle files up to 4GB so if you have files larger than 4GB, you'll have to go another route.

http://www.mac-forums.com/forums/other-hardware-peripherals/179918-external-hard-drive.html - this is the best one if you have Snow Leopard

http://www.mac-forums.com/forums/switcher-hangout/178854-back-up-windows-os-x-one-hard-drive.html