Give this a try and let us know how you get on:
1. Plug the disk into a free USB port on your mac and open disk utility (in applications / utilities).
2. You should see that it auto mounts the disk and it appears in the left hand pane.
3. Assuming it does select the disk by single clicking on it. Note, not the partition underneath it!
4. Select the partition button on the right body of the application.
5. Select volume scheme and change it using the drop down from current to whatever number of partitions you want (1 for instance).
6. The greyed out format box should now be changeable.
7. Select the format you require and click apply.
There might be a compatibility issue between your operating system and external storage device. However, the best thing you can do to solve this problems is to safely format the storage device to a Windows or other operating system and use it on your iMac OS computer.