In the last few weeks, a few have mentioned an old procedure used in classic Mac OS to ‘rebuild the Desktop’. In those days, Mac OS maintained hidden databases that matched your apps to the documents they could open, using two four-character codes on every file setting its type and creator. Periodically, this would break down and all your document icons were replaced with generic forms, and the links between documents and apps broke. You then had to maintain and rebuild those databases before the Finder worked properly again.

While macOS thankfully no longer uses this mechanism, but has UTIs and LaunchServices databases instead, those old type and creator codes are still recognised, and macOS can use them to work out the UTI of a file using them, when there’s no other way.

Before you accuse me of April Foolery, the article below has been reformatted from an original first published in MacUser volume 17 issue 07, 2001. It’s a faithful account of routine maintenance we had to perform then on Mac OS.

What it does

The Desktop files are hidden database files used by the Finder to create the icons that make up the illusion of your Mac’s Desktop. Maintaining those databases ensures that icons remain properly recognised – a task you should do from time to time.

What you need

Mac OS, Disk First Aid, optional third party disk tool such as TechTool Pro.

Potential problems

Insufficient disk space to build and save new Desktop database files causes fatal errors, as will a write-protected disk. Missing applications can leave document icons still in their ‘generic’ form, for which the only solution is to install an application which can open them. If you don’t carry out disk checks first (as recommended here), minor errors can readily grow into serious ones and cause crashes.

Further info

Most Mac books describe this, as does the built-in help system for Mac OS, accessible from the Finder’s Help menu.

Checklist

• Check your disk with Disk First Aid and perform repairs.
• Ensure there’s sufficient free disk space.
• Restart, holding down the Command and Option keys until the dialog appears.

Setting It Up

This is the principal reason for needing to rebuild your Desktop: instead of the documents (and even applications) being shown with their correct custom icons, they are displayed with these generic icons, or possibly even inappropriate ones. This is because the database files that the Finder uses to look up file types and the icons to be associated with them have fallen out of date, or become corrupted, and need to be rebuilt.

The Desktop files are hidden on each volume, and depend on your hard disk being sound and uncorrupted. It is best if you can run Disk First Aid (and any third party utility you have) to ensure that the volume whose Desktop you are going to rebuild is in good health. If any problems are found, be sure to fix them before you rebuild, otherwise you can turn a flakey disk into a completely broken one.

If you have a third party utility such as TechTool Pro or Norton Utilities, check the Finder information of the files on the volume in question. This will ensure that applications are correctly recognised as such, and during the rebuild they will be examined properly to see if they define custom icons.

The final step before starting the rebuild is to ensure that you have sufficient free space on the volume to accommodate the new database files. A rebuild temporarily requires a not insignificant amount of additional free space – it is impossible to quantify, but if the rebuild runs out of space, it will fail (and sometimes horribly, with a crashed disk). Select the volume icon and use the Finder’s File/Get Info menu command to check.

Once you’re ready to rebuild, restart your Mac and hold down the Command and Option keys until you see this dialog. Click on OK, and repeat this for each volume as it is being mounted by Mac OS. Some disk checkers, such as TechTool Pro, may cut in during this, but they should not cause any problems.

TechTool Pro offers special features for rebuilding, checking, repairing, and saving Desktop files, which can be used as an alternative. If you have performed a rebuild that does not seem to fix your problems, then you may find it helpful to run the Check/Repair Desktop tool. TechTool’s ability to fix problems in Desktop database files is a good enough reason to buy it, even if you prefer Norton for disk checking and repair.