usb media I/O error, disagree with fstab, system freeze

I have a USB storage media (jacqueslemaire) which, if left inactive for any period of time, will go into 'sleep' mode (to save energy), causing the kernel to lose touch with it. It is still officially mounted, but no I/O operation can be performed.
If I try to umount it, I get an error saying:
'mount status disagrees with fstab' (or something similar).
I do sudo umount, the keyboard starts to flash and no input device (keyboard, mouse) work at all. At this stage, only a hard reboot will restore control to the system.