An interesting problem; if I understand correctly, the laptop contains
the data, and the thumbdrive contains the key.  I've seen similar
systems, but usually involving an encrypted filesystem on the
thumbdrive, using a password-based key (mounting the thumbdrive to read
the data).

Can't think of a *simple* solution off the top of my head; one approach
(assuming Java-based, since your post was here) might be:

1) Install a standalone servlet container/web server on the laptop
(accessible only from localhost).

2) Customize/implement a WebDAV servlet (there are a few, Tomcat comes
with one) to serve up the "c:" drive (assuming a Win-based laptop).

3) Have your customized WebDAV servlet obtain the key from a specified
location (e.g., the thumbdrive mounted on "d:").

4) Have your customized WebDAV servlet decrypt/encrypt files on the fly
using the obtained key.

Effectively, the WebDAV servlet would act as a proxy to the hard drive,
decrypting file downloads and encrypting uploads.  You would then open
http://localhost/webdav using a WebDAV-enabled client (i.e., Windows web
folders) and copy files to/from using Explorer.

You could also implement a pseudo-filesystem inside a single file, and
serve that up (rather than "c:"); essentially all encrypted data would
reside in a single file, accessible through the WebDAV servlet.  This
would be a lot more work, but you could then burn the encrypted file
onto a CD and transfer it easily from system to system.  You could also
make the web server accessible remotely over HTTPS, so the user could
start the web server, insert the thumbdrive, and have remote clients
access the data securely over HTTPS (authenticated, perhaps via client
SSL certificates).

This is actually pretty interesting; I might look at implementing
something similar myself.

Eric