lord god you people, you're making this difficult.... First a couple of questions...
Dave1234... As was asked, is this an internal HD (like your system is running from it), external (like, one you use just for certain purposes), or a network HD?
Also, are you running OS X?
If you're running X, and it's an external drive, both are reasonable assumptions i think... Click on the drive's icon, go to File -> Get Info. Click the "Ownership & Permissions" triangle. The Owner should be your username, with Read & Write access, if not click the Unlock button, authenticate, and change it.
You should also repair permissions (this might automatically do the above steps)... Open the program "Disk Utility" in your /Applications/Utilities/ folder, click the "First Aid" box, select your disk and click "Repair Disk Permissions". Should fix you right up.

pnw