pyboard, swapping sd cards

[rhubarbdog]

hi,
in my /flash directory i have a SKIPSD file
i mount my SD card as follows os.mount(pyb.SDCard(),'/sd')
using screen i enter a REPL session
I issue the command os.umount("/sd")
os.listdir('/') proves it's safe to remove the disk.
I extract it and put a new SD Card in.
When i issuse the command os.mount(pyb.SDCard(),'/sd') i get an OSError , error 19 ENODEV.

is it possible to swap SD cards whilst the pyboard is running? or does in need to be hard reset?