Wow, I'm overwhelmed, thank you very much for your help!
So, firstly I did a
dd if=/dev/zero of=nullz.raw bs=1 count=0 seek=2028060672
and overwrote the ISO on the HDD.
Then I was able to restore the GPT using gdisk with "use backup GPT
header (rebuilding main)". Now I have an intact GPT, a displayed
partition with original name, and an unknown filesystem.
btrfs check --repair /dev/sdc1
and
btrfs-show-super /dev/sdc1 --all
still don't work.
But I btrfs-show-super now finds the third superblock (see at the
bottom of this mail).
Then I tried btrfs restore with -u (superblock 3, because it's 0, 1, 2):
$ losetup /dev/loop1 -o 1M /dev/sdc
$ btrfs restore /dev/loop1 -l -u 2
checksum verify failed on 21004288 found E4E3BDB6 wanted 00000000
checksum verify failed on 21004288 found E4E3BDB6 wanted 00000000
checksum verify failed on 21004288 found E4E3BDB6 wanted 00000000
checksum verify failed on 21004288 found E4E3BDB6 wanted 00000000
bytenr mismatch, want=21004288, have=0
ERROR: cannot read chunk root
Could not open root, trying backup super
Hm, doesn't "-u 2" defines the backup superblock 3?
$ btrfsck /dev/loop1
No valid Btrfs found on /dev/loop1
Couldn't open file system
BTW, Photorecs finds lots of files but e.g. MKV video are corrupted in
some form that they don't want to play but KDE's Dolphin is able to
generate previews in some cases.
Any idea about how I should proceed further?
$ btrfs-show-super /dev/sdc1 --all
superblock: bytenr=65536, device=/dev/sdc1
---------------------------------------------------------
ERROR: bad magic on superblock on /dev/sdc1 at 65536
superblock: bytenr=67108864, device=/dev/sdc1
---------------------------------------------------------
ERROR: bad magic on superblock on /dev/sdc1 at 67108864
superblock: bytenr=274877906944, device=/dev/sdc1
---------------------------------------------------------
csum 0x615e669a [match]
bytenr 274877906944
flags 0x1
( WRITTEN )
magic _BHRfS_M [match]
fsid 446c7a9c-fcad-42f2-b093-ee495ca8f5be
label Speicherschatz
generation 5503
root 5113205555200
sys_array_size 129
chunk_root_generation 5485
root_level 1
chunk_root 21004288
chunk_root_level 1
log_root 0
log_root_transid 0
log_root_level 0
total_bytes 8001561821184
bytes_used 6272827662336
sectorsize 4096
nodesize 16384
leafsize 16384
stripesize 4096
root_dir 6
num_devices 1
compat_flags 0x0
compat_ro_flags 0x0
incompat_flags 0x169
( MIXED_BACKREF |
COMPRESS_LZO |
BIG_METADATA |
EXTENDED_IREF |
SKINNY_METADATA )
csum_type 0
csum_size 4
cache_generation 5503
uuid_tree_generation 5503
dev_item.uuid 0471bf89-89d1-424a-9fc1-d48241ff453b
dev_item.fsid 446c7a9c-fcad-42f2-b093-ee495ca8f5be [match]
dev_item.type 0
dev_item.total_bytes 8001561821184
dev_item.bytes_used 6283562319872
dev_item.io_align 4096
dev_item.io_width 4096
dev_item.sector_size 4096
dev_item.devid 1
dev_item.dev_group 0
dev_item.seek_speed 0
dev_item.bandwidth 0
dev_item.generation 0
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