I was working on one of these old iDRACs and once I added a new ISO, I was not able to mount it because of the error below.
iDRAC 6: Drive Error Either Virtual Media is detached or Virtual Media redirection for the selected virtual disk drive is already in use.
In order to fix this issue, connect to iDRAC with an admin account and go to System > Console/Media, then on Configuration.
Under “Virtual Media” change it to Attach (or Auto Attach if you like).
Apply the changes and you’re good to go!