My experience today with generic 1GB USB key on DS1054Z :
1) USB key formatted long time ago inserted on "powered on" DSO, not recognized.
2) Then I formatted the key FAT32 under Win7, still not recognized by DSO.
3) Then, I removed the key, I switched the DSO off, I inserted the key, I switched the DSO on, key was recognized immediately, as a small USB logo appeared in the down right corner. At this point, the key seems to be fully recognized as it appears in the Storage menu and saving a screenshot with the green "Printer" button works.
4) Now, I can remove and insert the USB back into the DSO without having to switch it off and on, the USB key is recognized immediately.
FWIW.