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Reader Richard Graham finds his Mac mini's hard drive minier than expected. He writes:

My Mac mini alerted me that my hard drive was almost full and I couldn't figure out how I had anything close to 55GB of accumulated junk. When searching the drive for large folders or files I found a folder in my user Library folder named "Mirrors." Nested inside was a disk image file (.dmg) that was over 15GB in size. Its file name matched my .Mac user name and appears to have been modified recently. I did a daily backup to my iDisk on .Mac (and now MobileMe) but nothing that approaches 15GB.

Any thoughts on its origin and is it safe to delete the file to free up hard drive space?

Your hinted suspicions are correct. That file is an artifact of the way the .Mac iDisk Sync feature worked. When you switched iDisk Sync on, a local copy of your iDisk was created on your hard drive in the location you mention--youruserfolder/Library/Mirrors. Currently, under Leopard and MobileMe, the disk image is no longer stored in a Mirrors folder. Instead, when you turn off iDisk Sync you'll see a disk image on your Desktop named Previous Local iDisk For yourusername.sparsebundle. You can double-click this image to mount a copy of your iDisk that was created before you last turned iDisk Sync off.

If you're running Leopard, the size of that copy shouldn't be too far out of line with the amount of space taken up on your iDisk out there in the cloud. If this copy was created under OS X 10.4 or 10.3, however, it will be the size of your entire iDisk--and a bit more. So, if your iDisk was 10GB, its size on your Mac will be around 12GB.

As for deletion, feel free to switch iDisk Sync off and then toss the disk image. If you see a warning that the file is busy when you try to empty the Trash, log out and back in again and then empty the Trash.