When I plugg my Garmin into the car to recharge, a picture of the garmin connected to a computer shows and the Garmin will not work. If I unplug the Garmin, it will work.

If this is a failure to recognize the device error, the following from a Garmin article may help:

All current Garmin devices, when connected to a computer, will communicate in either Mass Storage or Media Transfer Protocol (MTP) mode. If the computer is not detecting the Garmin device, there are a few troubleshooting steps that can be done.
Unplug the USB cable from the computer
Turn the device off and then back on again
Plug the USB cable directly into the device and the other end directly into a USB port on the computer and not a hub or docking station
The device should automatically go into Mass Storage or MTP mode, which is indicated by a picture of a GPS device connected to a tower on the Garmin screen.
To check the computer's recognition of the device on a Windows PC click on the Start menu and select (My)Computer. The device will show in this window if the computer recognizes it. On a Mac, the device will show as an icon on the desktop, labeled "GARMIN", or it can be found by selecting Finder from the dock and viewing the device list on the left-hand side menu within the window.
Note: If the computer still does not detect the device, try plugging it into a different USB port. If the problem persists, try another computer, if available. Ensure the USB cable is not connected to a cradle, if one is attached to the GPS.

Here is a link to the Garmin site itself address the issue of an unrecognized device:

https://support.garmin.com/en-US/?faq=PqkmoJbEgq7JqOKKZAkFK9&productID=69452&searchQuery=not%20recognized&tab=topics

And, the following information from another site may help:

https://www.gpscity.com/questions?qid=44804

You have not indicated the operating system of the computer. Sometimes you can use Garmin Express. Sometimes an older webupdater. It seems to depend upon the OS of the computer and that of the Garmin as to which to use. You may have to contact Garmin help directly if the above is insufficient.

The computer picture shows if the usb cable connects to a computer. If the picture shows when the usb socket is connected to the cigar lighter power supply, it could be that the socket in the Garmin has got damaged. I had a Nuvi 200 like this where the socket had broken away from the circuit board inside, but the repair was difficult, involving micro-soldering!

Hello,

When the Garmin shows the picture of a computer, it means that the Nuvi is in mass storage mode. This mode is used to load software and map updates to the Nuvi and will usually go into this mode when connected to a computer or improper USB cable.
If this is occurring when it is connected to the vehicle power cable, chances are that the vehicle power cable is the wrong one.
You may also want to contact Garmin Tech Support for assistance.
1-800-800-1020

Hello,

When the device shows that it is hooked up to a larger device, it is in mass storage or data mode and usually will do this when connected to a computer to receive map and software updates.
This will also happen if the Nuvi is connected with an incompatible cable.
Using the correct vehicle power cable will remedy this.
The Nuvi 300 is an old GPS and should probably be replaced with a newer model.

Try a different power cord. The unit thinks it's connected to a computer for some reason.

When it is plugged into the computer you cannot see if it is charged. You would have to unplug it from your computer after a few hours to let it charge, have it start up and then check the number of batter bars on the top right corner of your screen

My unit was totally dead. I just kept it plugged into my computer for about 12 hours. I downloaded the newest software from Garmin. When I unplugged it from my computer, the battery seemed to drain quick, but once I plugged it into the card adapter in the car... everything seems to be running great now.

There is nothing wrong with your gps unit so you can relax and prepare for your trip!

While your unit is connected to the computer via usb cable as in recharging, you cannot operate it as a gps at the same time.

You will find that once the usb cable is disconnected that your unit is working normally.

Enjoy yourself.

your disk might be corrupted. May be the only solution is to format, with all data lost.
Otherwise try to plugg another SD in Garmin, and see result.

Be carefull that when removing (disconnect) a SD device, it must be ejected properly, otherwise it may be corrupted if a write operation is ongoing.

Are you using the Garmin AC adapter or the Garmin car adapter? Using the wrong cable and the Garmin will think that you are trying to connect to a computer. Not all USB cable are alike.

When connected to your computer it thinks that you want to upload or donwload information.