From Office.Microsoft – http://office.microsoft.com/en-us/outlook/HA012276281033.aspx
If you work offline with Outlook or use Cached Exchange Mode with Microsoft Exchange Server, you can change the location of the local Offline Folders File (.OST) to a different folder or hard drive.
Change the location where your offline files are saved
- Exit Outlook.
- Click Start, and then click Control Panel.
- Double-click Mail, and then click E-mail Accounts.
- Click View or change existing e-mail accounts, and then click Next.
- Select the Microsoft Exchange Server account, and then click Change.
- Clear the Use Cached Exchange Mode check box.
- Click More Settings.
- Click the Advanced tab.
- Click Offline Folder File Settings.
- Click Disable Offline Use, and then click Yes in the dialog box that appears.
- Click Offline Folder File Settings.
- Click Browse, go to the location where you want to save your .ost file, and then click Open.
- Click OK.
- Select the Use Cached Exchange Mode check box.
- Click Next, and then click Finish.