Error code -50 hard drives won't eject

I am using three G-drives, a 4TB RAID drive, and two 3-TB G-drives. On both my Macbook and iMac I am having problems ejecting them. I get the message

"The disk wasn't ejected because one or more programs may be using it." even when all the programs on the computer are closed.

I also get error code -50

"The operation cannot be completed. An unexpected error occurred (error code -50) when I try to create a new folder or move files on 2 of the drives.

At first I thought the problem was my iMac which recently had it's internal drive replaced, but I have been using the hard drives with my Macbook and am getting the same problems so now I think it's a problem with the drives, but since it is happening with all 3 I'm not sure what's up. I ran disk utility and verified the discs and it said everything was ok.

All three drives are used to store video files and FCP 7 media for editing.

I cannot find any information on what could be causing this and would appreciate any suggestions.

Thanks.