Same problem here using 16.04 / kernel 4.4.0-42-generic (i686) and
external drive LaCie d2 Quadra, USB ID 059f:105e

I get random hangs under heavy traffic, during rsync from one external USB 
drive to another.
rsync just hangs during a transfer. Sometimes it recovers after a number of 
seconds, sometimes it never does and hangs there, possibly for hours.
dmesg contains then contains repeated error recovery sequences such as:

[ 2350.056247] usb 1-4.4: cmd cmplt err -2
[ 2350.128049] usb 1-4.4: reset high-speed USB device number 12 using ehci-pci
[ 2350.333548] scsi host8: uas_eh_bus_reset_handler success
[ 2381.032039] sd 8:0:0:0: [sdc] tag#1 uas_eh_abort_handler 0 uas-tag 2 
inflight: CMD 
[ 2381.032045] sd 8:0:0:0: [sdc] tag#1 CDB: Read(10) 28 00 95 5a 11 00 00 00 08 
[ 2381.032049] sd 8:0:0:0: [sdc] tag#0 uas_eh_abort_handler 0 uas-tag 1 
inflight: CMD 
[ 2381.032052] sd 8:0:0:0: [sdc] tag#0 CDB: Read(10) 28 00 00 00 00 00 00 00 20 
[ 2381.032079] scsi host8: uas_eh_bus_reset_handler start

Disabling uas worked the problem around. I still see some:

usb 1-4.4: reset high-speed USB device number 14 using ehci-pci

in dmesg but the disk recovers after the reset.

Additionally, disabling uas allows me to see the external disk SMART
data (including using Disks), something I couldn't do before and was
sorely missing in these circumstances!

The bottom line is that it seems under certain circumstances, uas is not
able to recover from errors.

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  Seagate external drive causes SCSI bus resets when UAS enabled

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