I recently moved from a PC to a Mac. I have an Apple Time Capsule and use Time Machine to do regular backups. I wanted to backup an old hard disc from my PC but it doesn't seem to work. Also, why can't I just add a file to the backup? It is a 500GB disc but I don't seem to be able to do what I want. Tim, Sheung Wan DQ: I understand how this can seem confusing. When you first run Time Machine, it will ask you which disk you want to use for backup. Once you have chosen the disk, you can go into System Preferences and click on Time Machine to make changes. One of the less-than-obvious things you can do is choose which disks you do not want to backup. The implication is that all other disks connected to your machine will be backed up. You must make certain that you have enough disk space on the Time Machine backup drive before you start, however. It is not possible to backup a Windows system. You can backup individual files or folders but not the system itself. Finally, you cannot really treat Time Machine like a disk but it is easy enough to select a folder into which you put files you would like to have backed up. If Time Machine is set to backup once an hour - which is the default setting - your file would then be part of Time Machine within an hour. I have some old computers and would be happy to give them away but I understand some organisations will not take a computer if it is 'too old'. What is too old? Can I give them a Mac or do they only take PCs? Name and address supplied DQ: This is a good question. If you consider that some of us were fooling about with computers more than 25 years ago, it is clear that not all the machines one might want to bequeath are worth giving away. Hong Kong's Crossroads Foundation (www. crossroads.org.hk), which describes itself as 'an intersection between those in need and those who can help', has clearly defined what it will accept. The organisation will take some old equipment but computers must: be a Pentium 3 500 MHz or above; contain 129MB (or more) of system memory; have a hard drive of 8GB or more; and contain a power supply. This sounds quite reasonable and a keyboard, mouse and monitor would be useful as well. Crossroads says it will accept Macintosh computers. I frequently upload dozens of pictures to a website for friends to download. I have been criticised for not packaging them all into a zip file. I know a little bit about compressing files, but is there a free version I can use? Charles, Sai Kung DQ: There are many free versions of such software and they all seem to work quite well. Examples include ZipGenius ( www.zipgenius.com ), which supports more than 20 compressed archived formats; 7-Zip ( www.7-zip.org ), an open-source software that claims a high-compression rate under the new 7z format; ZipitFast ( www.zipitfast.com ), which supports almost all compression formats; and Stuffit Expander 2009 (mysmithmicro.com), a free tool that allows you to expand compressed or encoded files.