This is probably how Acorn hard disk images look (?)

Storage/MassStorage/Formats/AcornADF.cpp

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+//
+//  AcornADF.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 31/01/2021.
+//  Copyright © 2021 Thomas Harte. All rights reserved.
+//
+
+#include "AcornADF.hpp"
+
+using namespace Storage::MassStorage;
+
+AcornADF::AcornADF(const std::string &file_name) : file_(file_name) {
+	// Only one sanity check: is the file a multiple of 256 bytes in size?
+	// [TODO: and larger than a floppy disk?]
+	const auto file_size = file_.stats().st_size;
+	if(file_size & 255) throw std::exception();
+}
+
+size_t AcornADF::get_block_size() {
+	return 256;
+}
+
+size_t AcornADF::get_number_of_blocks() {
+	return size_t(file_.stats().st_size) / 256;
+}
+
+std::vector<uint8_t> AcornADF::get_block(size_t address) {
+	file_.seek(long(address * 256), SEEK_SET);
+	return file_.read(256);
+}
+
+void AcornADF::set_block(size_t address, const std::vector<uint8_t> &contents) {
+	file_.seek(long(address * 256), SEEK_SET);
+	file_.write(contents);
+}

Storage/MassStorage/Formats/AcornADF.hpp

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+//
+//  AcornADF.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 31/01/2021.
+//  Copyright © 2021 Thomas Harte. All rights reserved.
+//
+
+#ifndef AcornADF_hpp
+#define AcornADF_hpp
+
+#include "../MassStorageDevice.hpp"
+#include "../../FileHolder.hpp"
+
+namespace Storage {
+namespace MassStorage {
+
+/*!
+	Provides a @c MassStorageDevice containing an Acorn ADFS image, which is just a
+	sector dump of an ADFS volume.
+*/
+class AcornADF: public MassStorageDevice {
+	public:
+		AcornADF(const std::string &file_name);
+
+	private:
+		FileHolder file_;
+
+		/* MassStorageDevices overrides. */
+		size_t get_block_size() final;
+		size_t get_number_of_blocks() final;
+		std::vector<uint8_t> get_block(size_t address) final;
+		void set_block(size_t address, const std::vector<uint8_t> &) final;
+};
+
+}
+}
+
+#endif /* AcornADF_hpp */