Can't write to remote drive

[TRexx]

A few weeks a go I installed a SATA 1Tb drive on my main PC  to replace an old USB  drive that was failing. The drive works fine and I set it up so I could share it with the other 2 PCs on my home.   Initially everything worked fine.   Any of the 3 PCs could read & write data on the new drive. But  yesterday I suddenly got a authorization error trying to write to it from the 2 PCs that don't own it.  They can read data from it and can write to the other 2 drives on that main PC (internal hard drive and another USB 1TB drive). 

The permissions for the share are the same as the other 2 drives -- Everyone can do Everything.  I tried deleteing the share and creating a new one.  I've rebooted all the PCs but the problem persists.  When I display the drive properties from the other PCs it says "Everyone can do Everything"

The specific errors I get vary, for instance when trying to create new folder I get:

"Unable to create folder "New Folder", Access is denied.

When I try to copy a file using Windows Explorer I get:

"Cannot copy foo.txt. Access is denied. Make sure the disk is not full or write-protected and that the file is not currently in use".

The drive is about 30% full.
Any ideas? 

Oops. forgot to mention all PCs are running Windows XP + SP3

[pxsant]

I'm sure you already looked for  this but it sounds like check box "Allow Network Users To Change My Files" may have been unchecked.

[TRexx]

GRRR.  In the Properties dialog, on the Security tab, I had checked all the boxes for "EVERYONE".  But there is also a "Permissions" box on the Sharing tab.  And that one only had the "Read" box checked.  Once I checked "Full Control" & "Change" the problem went away.

SO WHY THE &(*&(*&(  ARE THERE 2 PLACES TO SET PERMISSIONS? 

and

WHY DID IT SUDDENLY CHANGE.

I really, really , really hate Windows.