Formatting the drive with a standard disk format will not fix the issue either. You'd need a special tool to do a secure erase (thus also needing to repartition) and I don't know if one is included on the N900 by default. Additionally, fragmentation on NAND has zero impact on read speeds. I think you're imagining things, really. Well, more like you smeared writes across much more of the flash. The end result being that future writes will be somewhat slower.