Comment 24 for bug 1336541

On a fully updated F20 machine hdparm shows that TRIM is supported on this Corsair
USB stick but both "fstrim" and "mount -o discard" commands fail
I have replicated the problem using an Addonics eSATAp to USB3 adaptor and a modern SSD.
ioctl/FITRIM problem

Details
ja@paxos ~ 1$ uname -a
Linux paxos 3.15.10-200.fc20.x86_64 #1 SMP Thu Aug 14 15:39:24 UTC 2014
x86_64 x86_64 x86_64 GNU/Linux

[root@paxos:~]$ hdparm -I /dev/sdb
ATA device, with non-removable media
        Model Number: Voyager GTX
        Serial Number: FF1807470C0800110923
        Firmware Revision: S9FM01.7
        Transport: Serial, ATA8-AST, SATA 1.0a, SATA II
Extensions, SATA Rev 2.5, SATA Rev 2.6, SATA Rev 3.0
...
           * Data Set Management TRIM supported (limit 8 blocks)
...
------------------------------------------------------------------------
root@paxos:~]$ mount |grep sdb
/dev/sdb1 on /media/gtx type ext4 (rw,nosuid,nodev,noexec,noatime,seclabel,discard,data=ordered)
//-----------------------------------------------------------------
dmesg -ew
[Aug23 08:45] EXT4-fs (sdb1): mounting with "discard" option, but the device does not support discard
[ +0.000002] EXT4-fs (sdb1): mounted filesystem with ordered data mode. Opts: discard
[ +0.000004] SELinux: initialized (dev sdb1, type ext4), uses xattr
//-----------------------------------------------------------------
[root@paxos:~]$ cat /sys/block/sdb/queue/rotational
0
//-----------------------------------------------------------------
[root@paxos:~]$ fstrim -v /media/gtx
fstrim: /media/gtx: discard operation not supported.
//-----------------------------------------------------------------
[root@paxos:~]$ strace fstrim /media/gtx
...
stat("/media/gtx", {st_mode=S_IFDIR|0777, st_size=4096, ...}) = 0
open("/media/gtx", O_RDONLY) = 3
ioctl(3, FITRIM, 0x7fffee2fb700) = -1 EOPNOTSUPP (Operation not supported)
...

The latest references to the problem that I can find are given here
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1336541
http://askubuntu.com/questions/262154/trim-and-ssd-with-usb-3-0-enclosure-does-not-work-uasp-not-supported
but I can find no reference to a likely, timely solution