From the Finder menu bar, select Go ▹ Go to Folder... and copy the following line into the text box that opens:
A Finder window should open with an item selected. Press the key combination command-I to open the Info window. In the Sharing & Permissions section of that window, do you have read & write access? Is the item locked? What is its modification date?
I'm not the original poster here, but I have the same problem. I looked at the "com.apple.Preview.signatures.plist" file, and, re the questions above:
- I do have read/write access
- The item is not locked
- The mod date is 2/14/13 (today is 2/21/13), and the creation date is also 2/14/13.
It looks like the file got overwritten with a new file. I attempted to recreate my signature a few days ago (after the old ones disappeared). That may explain the overwrite, although that should have modified the file.
My take here is that Preview is randomly creating a new signatures.plist file, and the fix is to use time machine to recover the old one.