You could share any folder on your computer by installing Bitvise SSH Server and using it to set up one or more virtual accounts.
Server Setup
- Install Bitvise SSH Server, and click on Open Easy Settings once the setup finishes
- In the 1. Server settings tab, enable Automatically configure router (requires UPnP), and change the setting Open Windows Firewall to Open port(s) to any computer
- Under the 3. Virtual accounts tab, set up one or more accounts making sure to enable the Allow file transfer option, and the Allow full access for the Virtual filesystem layout

- Save and exit. Then make sure to set up the appropriate permissions for your USB stick. You could do that through Right click ==> Properties ==> Sharing ==> Advanced Sharing... ==> Permissions ==> Full control for Everyone

Client Access
You could use any FTP client to access the files from any device on the network. Just use the username and password of the virtual account created in step 3. You could use the Bitvise SSH client, or opt for WinSCP or FileZilla for their portability
Summary
Set up Bitvise SSH Server on the main computer and use any FTP client to access it:
- It is free for personal use only
- Runs on Windows
- Easy to use (3-step setup)
- Shares folders with read/write permissions
- Clients can be portable such as WinSCP or FileZilla. The server must be installed
OSX 10.6.8 (Snow Leopard) and above compatible
- Clients, such as FileZilla, can be installed on OSX