Not sure about that, but:
the device name is set on installing the specific driver the first time (first time plugged your N900 to PC).
After flashing you "created a new/different device", so the PC does not see the original ID and give it a new name, but the driver was already installed; so the standard (removable device) was used.

relative sure:
Just go ahead and change in Win Explorer, there is no internal settings in N900 for the name shown on PC. At least nothing you can/should modify (it may be inside the first (the boot) sector of the first partition).

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it is in
*)���KNokia N900 FAT32 ¾┬≠¬"À├