time machine backup of Mac-formatted USB stick impossible in Lion?

Hello. Since Lion, Time Machine refuses to backup my USB sticks. In Options, they appear as excluded from backups. I can take them off that exclusion list and hit Save, yet when I reopen Options, they are back on it and still excluded. I even erased and re-formatted them (Mac OS Extended (Journaled)), no change. (The USB Sticks are: 4.04 GB LaCie itsaKey Media; 4,04 GB Generic Flash Disk Media.)
All this applies to USB Sticks only, not to USB external volumes. These are excludable and integratable as expected.
I can't imagine backing up USB sticks is now generally impossible; would anyone have any idea what might cause this problem?

Thanks, best