Ok, i've done some testing. My revo was on low battery (red led), i've used Metroid Zero Mission (USA) on kingston class 4 microSD (provided with device).
Test A
1. I've run a game, made a save, in the first slot (A), just after you got morph ball and...
2. Turn revo off with power button
3. Turn revo on
4. Loaded Metroid Z. again
5. Save slot - EMPTY
6. Checked save file on my PC: save file 139kb
Test B
1. I've run a game, made a save, in the first slot (A), just after you got morph ball and...
2. Used shortcut for saving: * + START
2. Turn revo off with power button
3. Turn revo on
4. Loaded Metroid Z. again
5. Save slot - OK, save was there, like it should be
6. Turn revo off with power button
7. Turn revo on
8. Loaded Metroid Z. again
9. Save slot - EMPTY
10. Checked save file on my PC: save file 139kb
Test C
1. I've run a game, made a save, in the first slot (A), just after you got morph ball and...
2. Used shortcut for saving: * + START
2. Turn revo off with power button
3. Turn revo on
4. Loaded Metroid Z. again
5. Save slot - OK, save was there, like it should be
6. Used shortcut for saving: * + START
7. Turn revo off with power button
8. Turn revo on
9. Loaded Metroid Z. again
10. Save slot - OK, save was there, like it should be
11. Used shortcut for saving: * + START
12. Turn revo off with power button
13. Turn revo on
14. Loaded Metroid Z. again
15. Save slot - OK, save was there, like it should be
See a pattern here? When i was saving with shortcut *+START and then, turn revo off - save would be ok. I've done it few times, with same results.