Cheap and cheerful image recovery from a corrupted SD card that keeps unmounting itself

reader.py

# usage:
#   sudo python reader.py /dev/disk2s2 path-to-my-image.img
#
# keep re-inserting the card and hitting enter as instructed


import os.path
import sys


infile = sys.argv[1]
outfile = sys.argv[2]


with open(outfile, 'wb') as ofh:
    blocks_read = 0
    while True:
        try:
            ifh = open(infile, 'rb')
        except (IOError, OSError):
            raw_input('Failed to open. Reinsert and press enter.')
            continue
        try:
            ifh.seek(blocks_read * 512)
        except IOError:
            raw_input('Failed to seek. Reinsert and press enter.')
            continue
        try:
            d = ifh.read(512)
        except IOError:
            print('block {:06d}: skipped'.format(blocks_read))
            ofh.write('\00' * 512)
            # close the file to free the handle - if left open it holds the volume mounted
            # even if it has become unreadable. closing the file at this point allows it to
            # unmount if it thinks it needs to.
            ifh.close()
            try:
                os.stat(infile)
            except (IOError, OSError):
                raw_input('Failed to read. Reinsert and press enter.')
        else:
            print('block {:06d}: read'.format(blocks_read))
            ofh.write(d)
            ofh.write('\00' * (512 - len(d)))

        blocks_read += 1