I recently sold my Dell notebook computer and bought a Mac. I had lots of files on an external hard disk attached to the Dell. I have been able to connect the hard disk to my Mac, but I can only read files from it. I have about 20 gigabytes of free space but I can't write anything to it. Can you help? John Handy, Mid-Levels DQ: This is called a 'permissions' problem. Somewhere along the way, your old disk has decided your new Mac is not its master. It will allow files to be taken, but it will not allow new ones to be written. There are two things you can do. On your desktop, highlight the disk with your mouse and click 'Get Info' on the menu bar, under 'File'. Then click on 'Ownership & Permissions' to see who 'owns' the disk. It should tell you what you need to know. If that doesn't work, look in the Utilities folder inside your Applications folder for something called 'Disk Utility'. Double-click on that to run it and select the drive that is causing you headaches. There is a button that says 'Repair Disk Permissions'; click on that. It should fix your problem. Otherwise, you may need to get another hard disk. I want to organise my contacts. Is there something I can use to help me? I use a Windows notebook personal computer for work and a Mac at home. Is this set-up going to create problems? Name and address supplied DQ: You are certainly not alone when it comes to wanting an effective system for managing the names and addresses of friends and business partners. A product by California-based company Plaxo ( www.plaxo.com ) may be just what you need. Since 2003, Plaxo has provided a free online address-book service that automatically updates contact information. More than 15 million users worldwide store their data on Plaxo's servers. The great thing about the software behind Plaxo is it works on both Mac and Windows machines. Whenever somebody on your contacts list updates their information, the Plaxo software will automatically synchronise it for you - as long as your friends use Plaxo, too. Last month saw the release of Plaxo 3.0, a major upgrade that allows a user's online address book and calendar to synchronise with Microsoft, Google, Mac OS X, Yahoo!, AOL and mobile phones. This new version, currently in beta (beta.plaxo.com), makes simple what had been impossible: bridging disparate tools and services with automated, multi-way synchronisation - without complicated software. You may also try WebAsyst ( www.webasyst.net/contact-manager.htm ). It offers a family of Web-based applications, including a contact manager, a document depot, a mail master, a photo depot and quick notes. Best of all, it's free.