Click the problem VM
The reason for this anomaly is because the disc image is mounted within a VM and is being recognized by a snapshot. Until this snapshot is removed with the associated VM, the discrepancy refuses to clear and you are unable to remove the ISO file from the images list. Here is a recommended procedure to quickly resolve this issue. This guide also applies to floppy and hard disk images as well.
Remove the ISO
First, click the name of the virtual machine that used the ISO file in question, click the "Snapshots" button on the right-hand side and then restore the snapshot you recently created by clicking the snapshot name and pressing Ctrl+Shift+R. When prompted, uncheck the checkbox for "Create a snapshot of the current machine state" before clicking the "Restore" button.
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