I can get this consistently on a Dell Pentium/90 dumping from
wd0a and restoring to sd0a. 
Here's the traceback:

_ffs_blkpref(f871e100,3,2c1,10)
_ffs_blkpref(f871e100,3,8058,2,f8164e88)
_ffs_reallocblocks(f7bffe20,f871e100,f8ed1000,f86c2000,101000)
_ffs_cluster_write(f8c928c8,101000,0)
_ffs_write(f7bffee4,f871dec0,1000,f7bfff94,0)
_vn_write(f871dec0,f7bfff30,f86fe280,f7bf9388,f86ff300)
_write(f86ff300,f7bfff94,f7bfff8c,0,f7bf97b8)
_syscall

I hope that helps.

christos

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