Exact same problem.

When I hot unplug the HDD connected to the card, I get a kernel stack trace:
Jan 16 08:19:41 ubuntu0-l kernel: [  802.774356] usb 3-1: USB disconnect, device number 2
Jan 16 08:19:41 ubuntu0-l kernel: [  802.791914] ------------[ cut here ]------------
Jan 16 08:19:41 ubuntu0-l kernel: [  802.791918] kernel BUG at /build/buildd/linux-3.2.0/fs/btrfs/transaction.c:1337!
Jan 16 08:19:41 ubuntu0-l kernel: [  802.791920] invalid opcode: 0000 [#1] SMP  
Jan 16 08:19:41 ubuntu0-l kernel: [  802.791923] CPU 3  
Jan 16 08:19:41 ubuntu0-l kernel: [  802.791924] Modules linked in: nls_iso8859_1 nls_cp437 vfat fat nvidia(P) pci_stub vboxpci(O) vboxnetadp(O) vboxnetflt(O) vboxdrv(O) ebtable_nat ebtables ipt_MASQUERADE iptable_nat xt_CHECKSUM iptable_mangle bridge stp kvm_intel kvm joydev ip6t_LOG nf_conntrack_ipv6 nf_defrag_ipv6 ipt_REJECT ipt_LOG ipt_ULOG snd_hda_codec_hdmi xt_limit xt_tcpudp xt_addrtype xt_owner xt_conntrack arc4 ip6table_filter ip6_tables uvcvideo btusb xt_state videodev xt_helper v4l2_compat_ioctl32 nf_nat_tftp nf_conntrack_tftp nf_nat_irc nf_conntrack_irc nf_nat_ftp snd_hda_codec_conexant nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 snd_seq_midi nf_conntrack_ftp nf_conntrack iptable_filter snd_rawmidi ip_tables x_tables snd_hda_intel snd_hda_codec snd_hwdep dm_multipath snd_seq_midi_event snd_pcm thinkpad_acpi iwlwifi snd_seq psmouse snd_seq_device serio_raw snd_timer mac80211 snd wmi bnep soundcore nvram snd_page_alloc cfg80211 rfcomm mei(C) mac_hid bluetooth parport_pc ppdev tpm_tis lp parport binfmt_misc ex
Jan 16 08:19:41 ubuntu0-l kernel: t2 dm_crypt dm_raid45 xor dm_mirror dm_region_hash dm_log btrfs zlib_deflate libcrc32c usb_storage uas vesafb usbhid hid mmc_block sdhci_pci sdhci video firewire_ohci firewire_core crc_itu_t e1000e [last unloaded: nvidia]
Jan 16 08:19:41 ubuntu0-l kernel: [  802.791983]  
Jan 16 08:19:41 ubuntu0-l kernel: [  802.791985] Pid: 15589, comm: umount Tainted: P         C O 3.2.0-35-generic #55-Ubuntu LENOVO 4243EN1/4243EN1
Jan 16 08:19:41 ubuntu0-l kernel: [  802.791988] RIP: 0010:[<ffffffffa00ee8ff>]  [<ffffffffa00ee8ff>] btrfs_commit_transaction+0x82f/0x840 [btrfs]
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792005] RSP: 0018:ffff8801a7ccbd88  EFLAGS: 00010282
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792006] RAX: 00000000fffffffb RBX: ffff8801acdf4000 RCX: 0000000000000000
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792008] RDX: ffff8801a7ccbc90 RSI: 0000000001e95000 RDI: ffff8801acdf4060
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792009] RBP: ffff8801a7ccbe18 R08: 0000000000002000 R09: 0000000000000000
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792011] R10: 0000000000000000 R11: 0000000000000001 R12: ffff8801acdf4078
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792013] R13: ffff8801b6d04800 R14: ffff880202f8b000 R15: 0000000000000000
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792015] FS:  00007f9e538f5800(0000) GS:ffff88021dcc0000(0000) knlGS:0000000000000000
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792016] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792018] CR2: 00007f8d42a870e0 CR3: 00000001ae3e6000 CR4: 00000000000406e0
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792020] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792022] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792024] Process umount (pid: 15589, threadinfo ffff8801a7cca000, task ffff8801b72ac500)
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792026] Stack:
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792027]  0000000000000000 0000000000000001 0000000000000000 ffff8801b72ac500
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792030]  ffffffff8108bd20 ffff8801a7ccbdb0 ffff8801a7ccbdb0 00000000a0107cbb
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792033]  ffff8801a7ccbe58 ffff8801b6d04800 0000000000000001 ffff88020d9f3cc0
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792037] Call Trace:
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792042]  [<ffffffff8108bd20>] ? add_wait_queue+0x60/0x60
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792052]  [<ffffffffa00c923c>] btrfs_sync_fs+0x6c/0xf0 [btrfs]
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792057]  [<ffffffff811a7dc3>] __sync_filesystem+0x63/0x90
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792061]  [<ffffffff811a7e6b>] sync_filesystem+0x4b/0x70
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792065]  [<ffffffff8117b3cb>] generic_shutdown_super+0x3b/0xe0
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792068]  [<ffffffff8117b506>] kill_anon_super+0x16/0x30
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792071]  [<ffffffff8117bb4c>] deactivate_locked_super+0x3c/0xa0
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792074]  [<ffffffff8117c3ce>] deactivate_super+0x4e/0x70
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792078]  [<ffffffff81198a8d>] mntput_no_expire+0x9d/0xf0
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792081]  [<ffffffff81199dbb>] sys_umount+0x5b/0xd0
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792084]  [<ffffffff816655c2>] system_call_fastpath+0x16/0x1b
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792085] Code: e9 90 f9 ff ff 0f 0b 0f 0b 0f 0b 0f 0b 0f 0b 0f 0b be fc 04 00 00 48 c7 c7 58 3b 14 a0 e8 5a 96 f7 e0 e9 48 fb ff ff 0f 0b 0f 0b <0f> 0b 66 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5  
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792113] RIP  [<ffffffffa00ee8ff>] btrfs_commit_transaction+0x82f/0x840 [btrfs]
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792124]  RSP <ffff8801a7ccbd88>
Jan 16 08:19:41 ubuntu0-l kernel: [  802.792129] ---[ end trace 0fed03856500c1a8 ]---