diff --git a/src/Makefile.test.include b/src/Makefile.test.include
index 839224e9a1c3c..a9b24c0007fde 100644
--- a/src/Makefile.test.include
+++ b/src/Makefile.test.include
@@ -89,6 +89,7 @@ BITCOIN_TESTS =\
   test/blockencodings_tests.cpp \
   test/blockfilter_tests.cpp \
   test/blockfilter_index_tests.cpp \
+  test/blockmanager_tests.cpp \
   test/bloom_tests.cpp \
   test/bls_tests.cpp \
   test/bswap_tests.cpp \
diff --git a/src/node/blockstorage.cpp b/src/node/blockstorage.cpp
index 960759396f7e3..2c8cacccef8fc 100644
--- a/src/node/blockstorage.cpp
+++ b/src/node/blockstorage.cpp
@@ -790,19 +790,24 @@ bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex, const Consensus
     return true;
 }
 
-/** Store block on disk. If dbp is non-nullptr, the file is known to already reside on disk */
 FlatFilePos BlockManager::SaveBlockToDisk(const CBlock& block, int nHeight, CChain& active_chain, const CChainParams& chainparams, const FlatFilePos* dbp)
 {
     unsigned int nBlockSize = ::GetSerializeSize(block, CLIENT_VERSION);
     FlatFilePos blockPos;
-    if (dbp != nullptr) {
+    const auto position_known {dbp != nullptr};
+    if (position_known) {
         blockPos = *dbp;
+    } else {
+        // when known, blockPos.nPos points at the offset of the block data in the blk file. that already accounts for
+        // the serialization header present in the file (the 4 magic message start bytes + the 4 length bytes = 8 bytes = BLOCK_SERIALIZATION_HEADER_SIZE).
+        // we add BLOCK_SERIALIZATION_HEADER_SIZE only for new blocks since they will have the serialization header added when written to disk.
+        nBlockSize += static_cast<unsigned int>(BLOCK_SERIALIZATION_HEADER_SIZE);
     }
-    if (!FindBlockPos(blockPos, nBlockSize + 8, nHeight, active_chain, block.GetBlockTime(), dbp != nullptr)) {
+    if (!FindBlockPos(blockPos, nBlockSize, nHeight, active_chain, block.GetBlockTime(), position_known)) {
         error("%s: FindBlockPos failed", __func__);
         return FlatFilePos();
     }
-    if (dbp == nullptr) {
+    if (!position_known) {
         if (!WriteBlockToDisk(block, blockPos, chainparams.MessageStart())) {
             AbortNode("Failed to write block");
             return FlatFilePos();
diff --git a/src/node/blockstorage.h b/src/node/blockstorage.h
index 7f671fd5f8de4..1b919df908e29 100644
--- a/src/node/blockstorage.h
+++ b/src/node/blockstorage.h
@@ -47,6 +47,9 @@ static const unsigned int UNDOFILE_CHUNK_SIZE = 0x100000; // 1 MiB
 /** The maximum size of a blk?????.dat file (since 0.8) */
 static const unsigned int MAX_BLOCKFILE_SIZE = 0x8000000; // 128 MiB
 
+/** Size of header written by WriteBlockToDisk before a serialized CBlock */
+static constexpr size_t BLOCK_SERIALIZATION_HEADER_SIZE = CMessageHeader::MESSAGE_START_SIZE + sizeof(unsigned int);
+
 extern std::atomic_bool fImporting;
 extern std::atomic_bool fReindex;
 /** Pruning-related variables and constants */
@@ -185,6 +188,7 @@ class BlockManager
     bool WriteUndoDataForBlock(const CBlockUndo& blockundo, BlockValidationState& state, CBlockIndex* pindex, const CChainParams& chainparams)
         EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
+    /** Store block on disk. If dbp is not nullptr, then it provides the known position of the block within a block file on disk. */
     FlatFilePos SaveBlockToDisk(const CBlock& block, int nHeight, CChain& active_chain, const CChainParams& chainparams, const FlatFilePos* dbp);
 
     /** Calculate the amount of disk space the block & undo files currently use */
diff --git a/src/test/blockmanager_tests.cpp b/src/test/blockmanager_tests.cpp
new file mode 100644
index 0000000000000..a9ce570a3df5b
--- /dev/null
+++ b/src/test/blockmanager_tests.cpp
@@ -0,0 +1,39 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <chainparams.h>
+#include <node/blockstorage.h>
+#include <node/context.h>
+#include <validation.h>
+
+#include <boost/test/unit_test.hpp>
+#include <test/util/setup_common.h>
+
+// use BasicTestingSetup here for the data directory configuration, setup, and cleanup
+BOOST_FIXTURE_TEST_SUITE(blockmanager_tests, BasicTestingSetup)
+
+BOOST_AUTO_TEST_CASE(blockmanager_find_block_pos)
+{
+    const auto params {CreateChainParams(ArgsManager{}, CBaseChainParams::MAIN)};
+    BlockManager blockman {};
+    CChain chain {};
+    // simulate adding a genesis block normally
+    BOOST_CHECK_EQUAL(blockman.SaveBlockToDisk(params->GenesisBlock(), 0, chain, *params, nullptr).nPos, BLOCK_SERIALIZATION_HEADER_SIZE);
+    // simulate what happens during reindex
+    // simulate a well-formed genesis block being found at offset 8 in the blk00000.dat file
+    // the block is found at offset 8 because there is an 8 byte serialization header
+    // consisting of 4 magic bytes + 4 length bytes before each block in a well-formed blk file.
+    FlatFilePos pos{0, BLOCK_SERIALIZATION_HEADER_SIZE};
+    BOOST_CHECK_EQUAL(blockman.SaveBlockToDisk(params->GenesisBlock(), 0, chain, *params, &pos).nPos, BLOCK_SERIALIZATION_HEADER_SIZE);
+    // now simulate what happens after reindex for the first new block processed
+    // the actual block contents don't matter, just that it's a block.
+    // verify that the write position is at offset 0x12d.
+    // this is a check to make sure that https://github.com/bitcoin/bitcoin/issues/21379 does not recur
+    // 8 bytes (for serialization header) + 285 (for serialized genesis block) = 293
+    // add another 8 bytes for the second block's serialization header and we get 293 + 8 = 301
+    FlatFilePos actual{blockman.SaveBlockToDisk(params->GenesisBlock(), 1, chain, *params, nullptr)};
+    BOOST_CHECK_EQUAL(actual.nPos, BLOCK_SERIALIZATION_HEADER_SIZE + ::GetSerializeSize(params->GenesisBlock(), CLIENT_VERSION) + BLOCK_SERIALIZATION_HEADER_SIZE);
+}
+
+BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/validation.cpp b/src/validation.cpp
index efe4e2cfe2f7d..844ad7eb6e2b9 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -4705,7 +4705,18 @@ void CChainState::LoadExternalBlockFile(
                     }
                 }
             } catch (const std::exception& e) {
-                LogPrintf("%s: Deserialize or I/O error - %s\n", __func__, e.what());
+                // historical bugs added extra data to the block files that does not deserialize cleanly.
+                // commonly this data is between readable blocks, but it does not really matter. such data is not fatal to the import process.
+                // the code that reads the block files deals with invalid data by simply ignoring it.
+                // it continues to search for the next {4 byte magic message start bytes + 4 byte length + block} that does deserialize cleanly
+                // and passes all of the other block validation checks dealing with POW and the merkle root, etc...
+                // we merely note with this informational log message when unexpected data is encountered.
+                // we could also be experiencing a storage system read error, or a read of a previous bad write. these are possible, but
+                // less likely scenarios. we don't have enough information to tell a difference here.
+                // the reindex process is not the place to attempt to clean and/or compact the block files. if so desired, a studious node operator
+                // may use knowledge of the fact that the block files are not entirely pristine in order to prepare a set of pristine, and
+                // perhaps ordered, block files for later reindexing.
+                LogPrint(BCLog::REINDEX, "%s: unexpected data at file offset 0x%x - %s. continuing\n", __func__, (nRewind - 1), e.what());
             }
         }
     } catch (const std::runtime_error& e) {