multiple cdrom drive letters have to be continous in dos. means you probably mounted two cd rom drives and between those two you mounted a normal drive, eg:
mount c c:\
mount d d:\ -t cdrom
mount e e:\ (not possible in dos !)
mount f f:\ -t cdrom
you may get away with that, but cdrom drive numbers are not correctly assigned, so you might get problems with it if a game relies on that.

No. that isn't the reason. I mounted all hard-disks first, then my two cd drives. One of them is a virtual one. Could it be the reason?

Mounting a virtual cd-drive shouldnt be a problem. maybe you mounted the two cd drives and skipped a drive letter, like
mount f e:\ -t cdrom
mount h f:\ -t cdrom
the result would be the error mentioned above, cos drive letter g is skipped. Or just tell me how you mount the drives exactly, maybe you found a bug...

I found the reason and it is a very STUPID one IMHO. This is an extract of my dosbox.conf (before the fix):

[autoexec]
# Lines in this section will be run at startup.
mount c c:\
mount d d:\
mount e e:\
mount f f:\ -t cdrom
mount h h:\ -t cdrom

And this is after the fix:

[autoexec]
# Lines in this section will be run at startup.
mount c c:\
mount d d:\
mount e e:\
mount f f:\ -t cdrom
mount g g:\ -t cdrom
mount h h:\ -t cdrom

As you can see, i have three cd-rom drives: F: G: and H: - F is a dvd-rom drive, G is a burner driver and H is a virtual one. Just because i usually don't use G (the burner) under DOSBox i wasn't mounting it. The error comes when drive letters do not match a proper letter sequence, because i was jumping from F to H. Even with the error message, there seems to be no impact in using DOSBox.

there is "Disable Smilies in this post" option so you can edit the post and set it ;)

You can solve your problem this way:
mount c c:\
mount d d:\
mount e e:\
mount f f:\ -t cdrom
mount g h:\ -t cdrom
#mount h g:\ -t cdrom

Thanks. 😎

I've just started using dosbox and have had the same error crop up and I'm only trying to install 1 cdrom drive

I currently have drive c mounted as c:\games\wc2

Then when I type:

mount d e:\ -t cdrom -ioctl

I get the above error message followed by:-

Drive D is mounted as CDRom

When I type mount it lists as being mounted but Warcraft II will still not recognise it. Any ideas?