I would like a way to get a number or string which identifies the server disk which hold my (Foxpro) database.

If I had such a number I could incorporate it into my credit card number encryption
so that even if the entire system was copied and brought up on a foreign machine
with a valid login the decryption still would not work.

The down-side of this approach is that it would mean unrecoverable data if the server machine ever dies.

Use Craig Boyd's encryption routines to encrypt the data:
http://www.sweetpotatosoftware.com/SPSBlog/PermaLink,guid,db662a8f-d47c-46c8-b0d2-a591c20d024b.aspx


Just store the encryption key in the registry somewhere for a simple way of storing the key away from the data.