We just had similar problems in another forum.
User tried for days on end only to find out his SD was faulty.
Also a lot of the 2 and 4GB noname cards are of very poor quality, resulting in a device that seems to work fine on the PC but fails everywhere else.
If you updated your 300 through Home you still have a very old Navcore running and most likely a bootload below 5.5012.
This might prevent you from using 4GB and SDHC SD cards.
For a 300 I recommend the 5.5112 bootloader.
If you want to have a full featured Navcore that can use the maps with ALG and IQ routes you can check my thread for the 9.485.
The version with memoryboost will work fine on your device.