Can't use External HardDrive

I have a SimpleTech - 160G external HD.
I've used this on a Windows Vista and MacBook before.
When I try to add or delete a file to the HD, this message is displayed:
"The item "________" could not be moved because "SignatureMini" cannot be modified."

Any suggestions?
I heard that possibly re-formatting the HD using disc utilities could work, but it's worked on my MacBook before.
As well, doing this could erase everything on the HD, and I do not have enough storage on my computer to store all the files.

Any suggestions?
( If I could backup all files for the External HD, how would this process be done? )​