When you try to delete a partition from a broken mirror set, you receive:
The drive cannot be locked for exclusive use. Please check to see if some applications are currently accessing the drive. If so, close them and try again.
To work around this problem:
01. Select the broken partition of the mirror set in Disk Administrator.
02. On the Tools menu, select Assign Drive Letter.
03. Press Do not assign a drive letter in the Assign Drive Letter dialog.
04. Press OK.
05. Use the Partition menu to Commit Changes Now.
06. Shutdown and restart your computer.
07. Delete the partition in Disk Administrator.
When you replace the failed drive, recreate the mirror.