xfs_copy stuck at 90% (sleeping process)

Hi fellow brothers,

It's the 474548735th time I try to do an offline backup using xfs_copy. Is it supposed to be efficient to clone filesystems between them, but...

Origin: 320GiB LVM2 snapshot of a XFS backup pool (301 GiB of 400 GiB used) Destination: 465GiB XFS in a USB disk

# time xfs_copy -b /dev/mapper/vg_poolbackup-snapbackup /dev/sdc1 0% ... 10% ... 20% ... 30% ... 40% ... 50% ... 60% ... 70% ... 80% ... 90% 

stack process is:

[<ffffffff810f135f>] futex_wait_queue_me+0xcf/0x130 [<ffffffff810f18e0>] futex_wait+0x180/0x280 [<ffffffff810f3b65>] do_futex+0xd5/0x4e0 [<ffffffff810f3fe1>] SyS_futex+0x71/0x150 [<ffffffff817baab2>] entry_SYSCALL_64_fastpath+0x16/0x75 [<ffffffffffffffff>] 0xffffffffffffffff 

Kernel is 4.2.0-22, Ubuntu 14.04 LTS

I tried xfs_copy some months ago with two little logical volumes (2 and 20 GiB), and worked fine. They were in the same internal drive; should the bottleneck be in the USB?

Any ideas? Thanks a lot for reading!