EXFAT formatted USB stick - sharing permissions wrong - Read Only - Cannot Reformat - Help please!

EXFAT formatted USB stick - sharing permissions wrong - Read Only - Cannot Reformat - Help please!

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Hi.

Having various problems with a USB stick that is formatted in EX-FAT.

It was working fine for a long period.

Yesterday I lent it to a friend to put some things on it.

Now when I try and work on it, I get messages like this.

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Mac OS X can’t repair the disk “Disk Name"

You can still open or copy files on the disk, but you can’t save changes to files on the disk. Back up the disk and reformat it as soon as you can.

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It will also randomly eject the drive.

I have tried to reformat it multiple times, in mac and windows.

[I have never used it in windows previous to this occasion of needing to reformat].

In windows, it crashes the disk when I attempt to format with disk manager [I attempted this due to advice in an internet article].

In Mac , I cannot even try to reformat it, because for some reason the permissions on the USB stick have been set to "read only" and I cannot find any way to change them to read-write, or full permissions, or whatever is appropriate.

I have tried "erase" disk in disk utility. I know how to do a full reformat, but it simply will not allow me to do so.

All the options are greyed out.

Does anybody have any information or links they can point me to?
I've been searching around.

Thanks very much in advance for any advice received.

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Running latest mountain lion , 10.8.4, and windows 7 through VMware fusion.