If you are using ADB tools to work on your phone but having problems accessing the sdcard, then it isn’t enough to just unmount the drive from your pc.

To remedy this you must put your phone into “Charge Only” mode before connecting it to your pc. To do this you need to go to “Settings->Connect To PC->Default Connection Type” and select “Charge Only”. Other options here are “Disk Drive” and “USB Tethering”. Once you’ve done this you’ll be able to run “adb shell” and then your attempt to run “cd sdcard” will succeed.