I did not resolve this issue with the latest checkbox stack.

ubuntu@howzit:~$ time sudo checkbox-cli run -t file --transport-where /tmp/submition-usb3-xhci-hcd.tar.xz -f com.canonical.plainbox::tar com.canonical.certification::device com.canonical.certification::usb3/storage-server


-------------------------[ Test USB 3.0 or 3.1 ports ]--------------------------
ID: com.canonical.certification::usb3/storage-server
Category: com.canonical.plainbox::usb
... 8< -------------------------------------------------------------------------
--------------------
Removable devices currently mounted:
None
Removable devices currently not mounted:
/dev/sda1
/dev/sdc1
/dev/sdb1
--------------------
Found the following mounted usb partitions:
    /dev/sda1 : /tmp/tmpg0qjhkh1 : 5000000000 bits/s
    /dev/sdc1 : /tmp/tmp_9vm1720 : 5000000000 bits/s
    /dev/sdb1 : /tmp/tmpb8cvl9ct : 480000000 bits/s (Will not test it, speed is below 500000000 bits/s)
--------------------
/dev/sda1 (Total Data Size / iteration: 255.9662 MB):
        [Iteration 0] Average Speed: 19.3209
        Summary:
                Total Data Attempted: 255.9662 MB
                Total Time to write: 13.2481 secs
                Average Write Time: 13.2481 secs
                Average Write Speed: 19.3209 MB/s
/dev/sdc1 (Total Data Size / iteration: 255.9662 MB):
        [Iteration 0] Average Speed: 18.1189
        Summary:
                Total Data Attempted: 255.9662 MB
                Total Time to write: 14.1270 secs
                Average Write Time: 14.1270 secs
                Average Write Speed: 18.1189 MB/s
                --------------------------------
                Device Detected: SuperSpeed USB
                Disk does not use xhci_hcd.
------------------------------------------------------------------------- >8 ---
Outcome: job failed
Finalizing session that hasn't been submitted anywhere: checkbox-run-2021-08-03T12.56.19
==================================[ Results ]===================================
Saving results to /tmp/submition-usb3-xhci-hcd.tar.xz

real    0m39.079s
user    0m8.892s
sys     0m2.384s


ubuntu@howzit:~$ dpkg -l '*checkbox*' '*plainbox*'
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                                   Version                                     Architecture Description
+++-======================================-===========================================-============-=======================================================
ii  checkbox-ng                            1.14.0+git202107121505+pkg170~ubuntu20.04.1 all          CheckBoxNG test runner
un  plainbox                               <none>                                      <none>       (no description available)
ii  plainbox-provider-certification-server 0.55.0+git202107151407+pkg164~ubuntu20.04.1 all          Server Certification provider for Plainbox
ii  plainbox-provider-checkbox             0.59.0+git202107261500+pkg168~ubuntu20.04.1 arm64        CheckBox provider for PlainBox
ii  plainbox-provider-resource-generic     0.49.0+git202107121510+pkg109~ubuntu20.04.1 arm64        CheckBox generic resource jobs provider
ii  python3-checkbox-ng                    1.14.0+git202107121505+pkg170~ubuntu20.04.1 all          CheckBoxNG test runner (Python 3 library)
ii  python3-checkbox-support               0.51.0+git202107121524+pkg143~ubuntu20.04.1 all          collection of Python modules used by PlainBox providers
un  python3-plainbox                       <none>                                      <none>       (no description available)