That's not good enough.

One issue is swap.

But I suspect there are other in-use circumstances this won't catch.

Sometimes doing nothing is better, because it encourages people to be
paranoid and careful of bear the consequences.

> I've been told I'm an idiot but I like to believe that my idiocy level
> isn't much above the average.  Unfortunately, I've done the same stupid
> thing more than once, which may indicate that I'm wrong.  And that's why I
> think the patch below may help me not loosing data by detaching an
> encrypted volume that has partitions mounted.  Which has happen.  Several
> times.  (Not say how many.)
> 
> Cheers,
> -- 
> Luís
> 
> diff --git sbin/bioctl/bioctl.8 sbin/bioctl/bioctl.8
> index a187daf36edd..6dc06b8503d4 100644
> --- sbin/bioctl/bioctl.8
> +++ sbin/bioctl/bioctl.8
> @@ -245,6 +245,9 @@ they become part of the array again.
>  .It Fl d
>  Detach volume specified by
>  .Ar device .
> +It will fail if
> +.Ar device
> +has a mounted partition.
>  .It Fl k Ar keydisk
>  Use special device
>  .Ar keydisk
> diff --git sbin/bioctl/bioctl.c sbin/bioctl/bioctl.c
> index 2e25d5801d9e..22281e2cce2d 100644
> --- sbin/bioctl/bioctl.c
> +++ sbin/bioctl/bioctl.c
> @@ -31,6 +31,8 @@
>  #include <sys/ioctl.h>
>  #include <sys/dkio.h>
>  #include <sys/stat.h>
> +#include <sys/types.h>
> +#include <sys/mount.h>
>  #include <dev/softraidvar.h>
>  #include <dev/biovar.h>
>  
> @@ -39,6 +41,7 @@
>  #include <fcntl.h>
>  #include <util.h>
>  #include <ctype.h>
> +#include <paths.h>
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <string.h>
> @@ -1080,6 +1083,23 @@ void
>  bio_deleteraid(char *dev)
>  {
>  	struct bioc_deleteraid	bd;
> +	struct statfs *fs;
> +	int len, n;
> +	char *s;
> +
> +	n = getmntinfo(&fs, MNT_NOWAIT);
> +	if (n == 0)
> +		errx(1, "getmntinfo");
> +	len = sizeof(_PATH_DEV) - 1;
> +	while (--n >= 0) {
> +		s = fs[n].f_mntfromname;
> +		if (strncmp(_PATH_DEV, s, len) == 0)
> +			s += len;
> +		if (strncmp(dev, s, strlen(dev)) == 0)
> +			errx(1, "%s is still mounted on %s",
> +			     fs[n].f_mntfromname, fs[n].f_mntonname);
> +	}
> +
>  	memset(&bd, 0, sizeof(bd));
>  
>  	bd.bd_bio.bio_cookie = bio_cookie;
>