The S.M.A.R.T. test verified my suspicion:

We run the background disk test which does not disturb the main operation:

      smartctl -t long /dev/ad5

We wait for finishing execution, viewing the status from time to time:

      smartctl -l selftest /dev/ad5

Finally, we look at the log contents, at the very end of the output:

      smartctl -a /dev/ad5

We have:

# 1 Extended offline Completed: read failure 90% 2916 10373954

We develop the complete list of bad sectors by reading all data from the disk:

   dd if=/dev/ad5 of=/dev/null bs=512 conv=noerror,sync

At some time we will see something like these:

   dd: /dev/ad5: Input/output error

   10373954+0 records in

   10373954+0 records out

   5311464448 bytes transferred in 2427.397393 secs (2188131 bytes/sec)

We will see the following in the system log:

   kernel: ad5: TIMEOUT - READ_DMA retrying (1 retry left)  LBA=10373954

   kernel: ad5: TIMEOUT - READ_DMA retrying (0 retries left) LBA=10373954

   ....

   kernel: ad5: FAILURE - READ_DMA timed out LBA=10374109

   kernel: ad5: TIMEOUT - READ_DMA retrying (1 retry left) LBA=10374113

We check each sector one more time:

   dd if=/dev/ad5 of=/dev/null bs=512 count=1 skip=10373954 conv=noerror,sync

We see which file in the file system comes within this block.

We see and roughly figure out the number of partition on which the bad sector falls (LBA 10373954):

   fdisk -s /dev/ad5

    /dev/ad5: 775221 cyl 16 hd 63 sec

   Part        Start        Size Type Flags

   1:          63   398444067 0xa5 0x80

   2:   398444130   382973535 0xa5 0x80

10373954 - 63 = 10373891

   disklabel /dev/ad5s1

    # /dev/ad5s1:

8 partitions:

   #        size   offset    fstype   [fsize bsize bps/cpg]

     a:  2097152        0    4.2BSD     2048 16384 28552 

     b:  4194304  2097152      swap                    

     c: 398444067        0    unused    0  0 # "raw" part

     d:  2097152  6291456    4.2BSD     2048 16384 28552 

     e: 10485760  8388608    4.2BSD     2048 16384 28552 

     f: 104857600 18874368    4.2BSD     2048 16384 28552 

     g: 104857600 123731968    4.2BSD     2048 16384 28552 

     h: 162127234 228589568    4.2BSD     2048 16384 28552 

We can see that 10373891 falls on the /dev/ad5s1e partition

We calculate the flushing relative to the beginning of the partition

10373891 - 8388608 = 1985283

We find inode, to which the specified block belongs:

fsdb -r /dev/ad5s1e

findblk 1985283

we repeat it for each bad sector:

(you should use use debugfs for Linux - http://smartmontools.sourceforge.net/BadBlockHowTo.txt)

We attempt to write data on the bad sector in order to activate the

remapping process on the disk.

    sysctl kern.geom.debugflags=16 # otherwise there will be dd: /dev/ad5: Operation not permitted)

    dd if=/dev/zero of=/dev/ad5 bs=512 count=1 skip=10373954 conv=noerror,sync

    sysctl kern.geom.debugflags=0

If the sector became readable after the write, everything is OK;

if not, the backup area on the drive is full; we try to bar

adjacent sectors from referencing to the file system:

man badsect

/dev/ad5s1e is mounted as /usr, we create the /usr/BAD directory and execute

  badsect /usr/BAD 1985283

  fsck -y /dev/ad5s1e

Useful ports:

/usr/ports/sysutils/recoverdm

/usr/ports/sysutils/diskcheckd