[knc1]

At last!
This turkey finally pasted its Q.A. testing:

Spoiler:

=== Normal output when both sizes equal
=== ending on full block or partial block

=== Src and Dst both 3 full blocks, test: c-w/r/c
Termination on block boundary. (0+0.0+0)
Blocks: 3, plus Bytes: 0, Total skips: 0, Total bytes: 12288
test01.dst: Before: 12288; After: 12288

=== Src and Dst both 3 full blocks, test: c
Termination on block boundary. (0+0.0+0)
Blocks: 3, plus Bytes: 0, Total skips: 3, Total bytes: 12288
test01.dst: Before: 12288; After: 12288

=== Src and Dst both 3-1/2 blocks, test: c-w/r/c
Termination on partial block boundary; w/r/c (0+1.0+1)
Blocks: 3, plus Bytes: 2048, Total skips: 0, Total bytes: 14336
test02.dst: Before: 14336; After: 14336

=== Src and Dst both 3-1/2 blocks, test: c
Termination on partial block boundary; match (0+1.0+1)
Blocks: 3, plus Bytes: 2048, Total skips: 4, Total bytes: 14336
test02.dst: Before: 14336; After: 14336

=== Following are "Notice"s because they might be intended usage.
=== That is: Changing only an initial portion of the destination.

=== Src 2 full blocks, Dst 3 full blocks, test: c-w/r/c
Notice: EOF on test03.src, full block on test03.dst (0+0.1+0)
Blocks: 2, plus Bytes: 0, Total skips: 0, Total bytes: 8192
test03.dst: Before: 12288; After: 12288

=== Src 2 full blocks, Dst 3 full blocks, test: c
Notice: EOF on test03.src, full block on test03.dst (0+0.1+0)
Blocks: 2, plus Bytes: 0, Total skips: 2, Total bytes: 8192
test03.dst: Before: 12288; After: 12288

=== Src 2-1/2 blocks, Dst 3-1/2 blocks, test: c-w/r/c
Notice: Partial block on test04.src, common bytes: 2048; w/r/c (0+1.1+0)
Blocks: 2, plus Bytes: 2048, Total skips: 0, Total bytes: 10240
test04.dst: Before: 14336; After: 14336

=== Src 2-1/2 blocks, Dst 3-1/2 blocks, test: c
Notice: Partial block on test04.src, common bytes: 2048; match (0+1.1+0)
Blocks: 2, plus Bytes: 2048, Total skips: 3, Total bytes: 10240
test04.dst: Before: 14336; After: 14336

=== Src 3-1/4 blocks, Dst 3-1/2 blocks, test: c-w/r/c
Termination on partial block boundary; w/r/c (0+1.0+1)
Blocks: 3, plus Bytes: 1024, Total skips: 0, Total bytes: 13312
test05.dst: Before: 14336; After: 14336

=== Src 3-1/4 blocks, Dst 3-1/2 blocks, test: c
Termination on partial block boundary; match (0+1.0+1)
Blocks: 3, plus Bytes: 1024, Total skips: 4, Total bytes: 13312
test05.dst: Before: 14336; After: 14336

=== Src 3 blocks, Dst 2-3/4 blocks, test: c-w/r/c
Notice: Partial block on test06.dst, common bytes: 3072; w/r/c (1+0.0+1)
Blocks: 2, plus Bytes: 3072, Total skips: 0, Total bytes: 11264
test06.dst: Before: 11264; After: 11264

=== Src 3 blocks, Dst 2-3/4 blocks, test: c
Notice: Partial block on test06.dst, common bytes: 3072; match (1+0.0+1)
Blocks: 2, plus Bytes: 3072, Total skips: 3, Total bytes: 11264
test06.dst: Before: 11264; After: 11264

=== Typically error conditions (source and destination size mis-match).
=== But since EOF is only recognized when encountered, the destination
=== blocks prior to the current block have been modified.

=== Src 3 blocks, Dst 2 blocks, test: c-w/r/c
Notice: full block on source, EOF on test07.dst (1+0.0+0)
Blocks: 2, plus Bytes: 0, Total skips: 0, Total bytes: 8192
test07.dst: Before: 8192; After: 8192

=== Src 3 blocks, Dst 2 blocks, test: c
Notice: full block on source, EOF on test07.dst (1+0.0+0)
Blocks: 2, plus Bytes: 0, Total skips: 2, Total bytes: 8192
test07.dst: Before: 8192; After: 8192

=== Src 3 blocks, Dst 3-1/4 blocks, test: c-w/r/c
Notice: EOF on test08.src, partial block on test08.dst (0+0.0+1)
Blocks: 3, plus Bytes: 0, Total skips: 0, Total bytes: 12288
test08.dst: Before: 13312; After: 13312

=== Src 3 blocks, Dst 3-1/4 blocks, test: c
Notice: EOF on test08.src, partial block on test08.dst (0+0.0+1)
Blocks: 3, plus Bytes: 0, Total skips: 3, Total bytes: 12288
test08.dst: Before: 13312; After: 13312

=== Src 2 blocks, Dst 3 blocks, test: c-w/r/c
Notice: EOF on test09.src, full block on test09.dst (0+0.1+0)
Blocks: 2, plus Bytes: 0, Total skips: 0, Total bytes: 8192
test09.dst: Before: 12288; After: 12288

=== Src 2 blocks, Dst 3 blocks, test: c
Notice: EOF on test09.src, full block on test09.dst (0+0.1+0)
Blocks: 2, plus Bytes: 0, Total skips: 2, Total bytes: 8192
test09.dst: Before: 12288; After: 12288

=== Src 3-1/2 blocks, Dst 3 blocks, test: c-w/r/c
Notice: partial block on source, EOF on test010.dst (0+1.0+0)
Blocks: 3, plus Bytes: 0, Total skips: 0, Total bytes: 12288
test010.dst: Before: 12288; After: 12288

=== Src 3-1/2 blocks, Dst 3 blocks, test: c
Notice: partial block on source, EOF on test010.dst (0+1.0+0)
Blocks: 3, plus Bytes: 0, Total skips: 3, Total bytes: 12288
test010.dst: Before: 12288; After: 12288

Command shell scripting is slow, but programming flash erase blocks is even slower -
So the script first compares each source and destination block pair (or fractional common parts) and just skips to the next block if the same.

The attached tar-ball has it all -
dd and dc3dd for both x86 and armel
the copycmp.sh script
and the tests directory with reference input files (made with dc3dd).

Tomorrow - write up directions on how to use this invention.
(The kindle's updater might get stuck in a loop on errors, but this script will not - and will tell us more than we probably wanted to know.)

Note:
Does copycmp file-to-device, device-to-file, file-to-file, and I suppose device-to-device (that last use is untested).