If you use the volume restore instead of the restore to a folder in Retrospect, the invisible 'Icon' file with the custom icon gets removed and your drives.
You should use the "Retrieve files and folders" command instead of the "Restore Entire Disk" command in Retrospect.
The error you describe can also occur if you try to paste a custom icon on a volume icon.
To restore your custom icons, use ResEdit or some other resource utility, make an "Icon" file visible from another drive and, as a precaution, duplicate it. Copy the duplicate file to the Apple RAID drive. When the copy is complete, use ResEdit or other resource utility to make both the original and copied files invisible again (the Icon file must be invisible). Restart, and you should be able to create a custom icon.
Article Change History:
07 Jun 1996 - Updated article with more information.