I have a situation that may or may not be related - but in any case it
is about confusing different instances of the "same" device.

$ mount
...
/dev/sd0a on /mnt/sd0 type ffs (soft dependencies, local)
/dev/sd0j on /mnt/sd0/scratch type ffs (soft dependencies, local)

sd0 is acutally a USB mass storage that I apparently unplugged before
unmounting. Now it is stuck. I can't even re-mount the thing on the same
directory:

umass0 at uhub4 port 5 configuration 1 interface 0
umass0: DMI WD3000JB-00KFA0, rev 2.00/2.43, addr 2
umass0: using SCSI over Bulk-Only
scsibus0 at umass0: 2 targets, 1 lun per target
sd0 at scsibus0 target 0 lun 0: <DMI, WD3000JB-00KFA0, 2.43> disk fixed
sd0: 279 GB, 16383 cyl, 16 head, 63 sec, 512 bytes/sect x 586072368
sectors
$ sudo mount /mnt/sd0
mount: realpath /mnt/sd0: Input/output error

If I try to mount it on another path, say /mnt2/sd0, then I get funny
effects with /mnt2/sd0/scratch:

$ sudo mount /dev/sd0a /mnt2/sd0 

$ sudo mount /dev/sd0j /mnt2/sd0/scratch
mount: realpath /mnt2/sd0/scratch: Input/output error

$ ls /mnt2/sd0/
ls: scratch: Input/output error
altroot/        etc/            libexec/        proc/           tmp/
bin/            etc.old/        mnt/            rescue/         usr/
boot            gennetbsd*      mnt2/           root/           var/
cdrom/          gennetbsd202*   netbsd*         sbin/           vol1/
cdrom1/         home/           netbsd.old*     stand/
dev/            kern/           netbsd.old2*    sys@
emul/           lib/            p/              tftpboot/

so it still has some record of the mount point /scratch on sd0a, even
though it won't recognise *this* disk as the same sd0 in other
situations.

Mounting sd0j onto some other directory seems to work however.

-Olaf.
-- 
___ Olaf 'Rhialto' Seibert      -- You author it, and I'll reader it.
\X/ rhialto/at/xs4all.nl        -- Cetero censeo "authored" delendum esse.