If an md you're trying to unconfigure is part of a ccd, the attempt will hang in an uninterruptible syscall mddestroy until all of the ccds involving the md are unconfigured:
load: 2.10 cmd: mdconfig 54293 [mddestroy] 6.10r 0.00u 0.00s 0% 2384k
load: 1.84 cmd: mdconfig 54293 [mddestroy] 43.38r 0.00u 0.00s 0% 2384k
load: 1.66 cmd: mdconfig 54293 [mddestroy] 184.61r 0.00u 0.00s 0% 2384k
Worse, while this is happening, any attempt to use mdconfig (even to just list the configured mds) will crash (segmentation fault).
The good news is, once the ccds are all destroyed, the hanging mdconfig will exit nicely.
But it, probably, should fail immediately with an EBUSY or equivalent...