Take a look at dmesg . In my case, this was due to a hardware error: bad sector on my disk. :(
This may explain why the command worked after copying the files.
You should find something like this:
[ 1387.312569] ata3.00: exception Emask 0x0 SAct 0x800 SErr 0x0 action 0x0 [ 1387.314589] ata3.00: irq_stat 0x40000008 [ 1387.316618] ata3.00: failed command: READ FPDMA QUEUED [ 1387.318638] ata3.00: cmd 60/08:58:48:1e:54/00:00:12:00:00/40 tag 11 ncq 4096 in res 41/40:00:4b:1e:54/00:00:12:00:00/40 Emask 0x409 (media error) <F> [ 1387.322675] ata3.00: status: { DRDY ERR } [ 1387.324690] ata3.00: error: { UNC } [ 1387.363352] ata3.00: configured for UDMA/133 [ 1387.363369] sd 2:0:0:0: [sda] tag#11 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE [ 1387.363375] sd 2:0:0:0: [sda] tag#11 Sense Key : Medium Error [current] [descriptor] [ 1387.363379] sd 2:0:0:0: [sda] tag#11 Add. Sense: Unrecovered read error - auto reallocate failed [ 1387.363384] sd 2:0:0:0: [sda] tag#11 CDB: Read(10) 28 00 12 54 1e 48 00 00 08 00 [ 1387.363388] blk_update_request: I/O error, dev sda, sector 307502667 [ 1387.365427] ata3: EH complete
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