GPS unit is stuck on the main screen, and wont turn off

This means it is not reading the SD card and that it needs to be re-formatted.

This is best done in a card reader and not the tomtom itself.
Right click on it and select format select FAT32 as the files system
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Now connect the tomtom but don't let tomtom home run. exit TTH if need be.
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You now ned to select your tom tom which will also show as a drive.
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In windows explorer select tools then folder options. Click view and tick the box for "show hidden files and folders"
It may warn you but you can ignore it.
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Then just copy ALL the files from the tom tom to your sd card
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DO NOT USE CUT AND PASTE !
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Once all the files are on switch off the tom tom and insert the sd card
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It should now work