What data are you trying to save? If it's on the media card, there's no issue. If it's in MyDocs, then reflashing the firmware won't affect it (nor will it affect contacts, calendar, etc.) - you will lose the installed applications (but shouldn't lose their data/settings). If you're reflashing the eMMC as well, then you'll lose all your settings, plus everything on MyDocs. In this case, prior to PR1.2 you can connect the powered-off N900 to your PC and it'll start up in mass storage mode, allowing you to copy the data from MyDocs. This functionality has been removed in PR1.2 (as it's a possible security issue), so I'm not sure what you can do there. Possibly you can start the phone in R&D mode - this may allow you to make backups and/or copy data off (I've not tried myself though).