diff --git a/Sources/ContainerizationEXT4/EXT4+Formatter.swift b/Sources/ContainerizationEXT4/EXT4+Formatter.swift
index 03bd304c0..6d2cabd7c 100644
--- a/Sources/ContainerizationEXT4/EXT4+Formatter.swift
+++ b/Sources/ContainerizationEXT4/EXT4+Formatter.swift
@@ -1434,6 +1434,7 @@ extension EXT4 {
             case internalInconsistency(_ message: String)
             case invalidBlockSize(_ size: UInt32)
             case journalTooSmall(_ size: UInt64)
+            case journalTooLarge(_ size: UInt64)
             public var description: String {
                 switch self {
                 case .notDirectory(let path):
@@ -1472,7 +1473,10 @@ extension EXT4 {
                     return "invalid block size \(size): must be 1024, 2048, or 4096"
                 case .journalTooSmall(let size):
                     return "requested journal size \(size) bytes is too small; minimum is \(EXT4.MinJournalBlocks) blocks (JBD2_MIN_JOURNAL_BLOCKS)"
+                case .journalTooLarge(let size):
+                    return "requested journal size \(size) bytes is too large"
                 }
+
             }
         }
 
diff --git a/Sources/ContainerizationEXT4/EXT4+Journal.swift b/Sources/ContainerizationEXT4/EXT4+Journal.swift
index 5e3511f6a..4c1179731 100644
--- a/Sources/ContainerizationEXT4/EXT4+Journal.swift
+++ b/Sources/ContainerizationEXT4/EXT4+Journal.swift
@@ -35,9 +35,16 @@ extension EXT4.Formatter {
             try self.seek(block: self.currentBlock + 1)
         }
         let journalStartBlock = self.currentBlock
+        let journalEndBlock = try calculateJournalEndBlock(
+            startBlock: journalStartBlock,
+            blockCount: journalBlocks
+        )
         try writeJournalSuperblock(journalBlocks: journalBlocks, filesystemUUID: filesystemUUID)
-        try zeroJournalBlocks(count: journalBlocks - 1)
-        try setupJournalInode(startBlock: journalStartBlock, blockCount: journalBlocks)
+        try skipJournalBlocks(count: journalBlocks - 1)
+        try setupJournalInode(
+            startBlock: journalStartBlock,
+            endBlock: journalEndBlock
+        )
         return journalBlocks
     }
 
@@ -51,8 +58,10 @@ extension EXT4.Formatter {
             guard blocks >= EXT4.MinJournalBlocks else {
                 throw EXT4.Formatter.Error.journalTooSmall(size)
             }
-            // Safe: any journal large enough to overflow UInt32 (>16 TiB at 4 KiB block size)
-            // would fail at the I/O layer before this conversion is reached.
+            // Safe: blocks is guaranteed to be at least EXT4.MinJournalBlocks and at most UInt32.max.
+            guard blocks <= UInt64(UInt32.max) else {
+                throw EXT4.Formatter.Error.journalTooLarge(size)
+            }
             return UInt32(blocks)
         }
         // Default sizing: scale with the usable content area, with a floor determined by
@@ -124,26 +133,19 @@ extension EXT4.Formatter {
         try self.handle.write(contentsOf: buf)
     }
 
-    private func zeroJournalBlocks(count: UInt32) throws {
+    private func skipJournalBlocks(count: UInt32) throws {
         guard count > 0 else { return }
-        let chunkSize = 1.mib()
-        // Safe: both operands are UInt32, so their product peaks at ~17 TiB, which fits
-        // in Int64 (the width of Int on all 64-bit Apple platforms).
-        let totalBytes = Int(count) * Int(self.blockSize)
-        let zeroBuf = [UInt8](repeating: 0, count: min(Int(chunkSize), totalBytes))
-        var remaining = totalBytes
-        while remaining > 0 {
-            let toWrite = min(zeroBuf.count, remaining)
-            try self.handle.write(contentsOf: zeroBuf[0..<toWrite])
-            remaining -= toWrite
-        }
+        // Widen before multiplying so the byte offset is calculated without UInt32 overflow.
+        let totalBytes = UInt64(count) * UInt64(self.blockSize)
+        try self.handle.seek(toOffset: self.pos + totalBytes)
     }
 
-    private func setupJournalInode(startBlock: UInt32, blockCount: UInt32) throws {
+    private func setupJournalInode(startBlock: UInt32, endBlock: UInt32) throws {
         var journalInode = EXT4.Inode()
         journalInode.mode = EXT4.Inode.Mode(.S_IFREG, 0o600)
         journalInode.uid = 0
         journalInode.gid = 0
+        let blockCount = endBlock - startBlock
         let size = UInt64(blockCount) * UInt64(self.blockSize)
         journalInode.sizeLow = size.lo
         journalInode.sizeHigh = size.hi
@@ -162,14 +164,19 @@ extension EXT4.Formatter {
 
         // Journal is one contiguous allocation → numExtents = 1 → extent tree fits inline
         // in the inode, so writeExtents needs no extra disk I/O for extent index blocks.
-        // Safe: blockCount is at most UInt32.max and startBlock ≥ 0, so the addition could
-        // theoretically overflow — but zeroJournalBlocks would have already failed with an
-        // I/O error if the journal extended past the end of the filesystem image.
-        journalInode = try self.writeExtents(journalInode, (startBlock, startBlock + blockCount))
+        journalInode = try self.writeExtents(journalInode, (startBlock, endBlock))
 
         self.inodes[Int(EXT4.JournalInode) - 1].pointee = journalInode
     }
 
+    private func calculateJournalEndBlock(startBlock: UInt32, blockCount: UInt32) throws -> UInt32 {
+        let (endBlock, overflow) = startBlock.addingReportingOverflow(blockCount)
+        guard !overflow else {
+            throw EXT4.Formatter.Error.journalTooLarge(UInt64(blockCount) * UInt64(self.blockSize))
+        }
+        return endBlock
+    }
+
     func journalInodeBlockBackup() -> (
         UInt32, UInt32, UInt32, UInt32, UInt32, UInt32, UInt32, UInt32,
         UInt32, UInt32, UInt32, UInt32, UInt32, UInt32, UInt32, UInt32,
diff --git a/Tests/ContainerizationEXT4Tests/TestEXT4Journal.swift b/Tests/ContainerizationEXT4Tests/TestEXT4Journal.swift
index 837032461..54694c518 100644
--- a/Tests/ContainerizationEXT4Tests/TestEXT4Journal.swift
+++ b/Tests/ContainerizationEXT4Tests/TestEXT4Journal.swift
@@ -163,4 +163,32 @@ struct JournalOverflowTests {
             try formatter.close()
         }
     }
+
+    @Test func closeRejectsJournalBlockCountOverflowingUInt32() throws {
+        let path = FilePath(
+            FileManager.default.temporaryDirectory
+                .appendingPathComponent(UUID().uuidString, isDirectory: false).path)
+        defer { try? FileManager.default.removeItem(atPath: path.string) }
+        let formatter = try EXT4.Formatter(
+            path,
+            journal: .init(size: (UInt64(UInt32.max) + 1) * 4096)
+        )
+        #expect(throws: EXT4.Formatter.Error.self) {
+            try formatter.close()
+        }
+    }
+
+    @Test func closeRejectsJournalEndBlockOverflowingUInt32() throws {
+        let path = FilePath(
+            FileManager.default.temporaryDirectory
+                .appendingPathComponent(UUID().uuidString, isDirectory: false).path)
+        defer { try? FileManager.default.removeItem(atPath: path.string) }
+        let formatter = try EXT4.Formatter(
+            path,
+            journal: .init(size: UInt64(UInt32.max) * 4096)
+        )
+        #expect(throws: EXT4.Formatter.Error.self) {
+            try formatter.close()
+        }
+    }
 }