Symptom
Mac OS X versions 10.0 to 10.0.4 allow you to execute the Paste command for a custom icon in several situations in which custom icons are not allowed and do not take effect. Symptoms of this issue appear in three ways:
1. A custom icon applied to a disk volume when started up with Mac OS 9 does not appear when started up with Mac OS X.
2. Mac OS X allows you to paste a custom icon on a disk volume, but it does not take effect. However, the custom icon is visible if you restart the computer with Mac OS 9.
3. Mac OS X allows you to paste a custom icon on a file that has no resource fork, but it has no effect. (Many files in Mac OS X do not have resource forks.)
Solution
Updating to Mac OS X 10.1 resolves part of Symptom 2, described above.
In Mac OS X 10.1, you can paste custom icons on a volume. However, the icon is stored in a different place than the icon used by Mac OS 9. This means you can have both a Mac OS 9 custom icon and a Mac OS X custom icon. The icon you see depends on whether the computer is started up from Mac OS 9 or Mac OS X.
Where custom icons are stored
The icon for a Mac OS X volume is stored in a file called ".VolumeIcon.icns " on the root level for that volume. This file is not normally visible in the Finder. The icon for the startup volume will be /.VolumeIcon.icns, while the icon for another disk called "disk2" will be stored at /Volumes/disk2/.VolumeIcon.icns.