I had issues initially but was able to fix it in case it helps others.
I am using the Samsung endurance micro SD card and the tiny SanDisk USB reader recommended a lot by others. I formatted it to fat32 and just had the TeslaCam folder in there but the car wouldn't recognize it. Turns out your not supposed to label the drive itself? Once I cleared the name of the drive and formatted it again then it works well.
Some drives or USB sticks will work with a name, some will not. For ease, reformat your media and don't name it. Once you do that, create a folder named TeslaCam (written just like that) and don't add anything else. Then go jack it in, hit your Sentry and Dashcam. Get some feed on there, and then disable them, pull the drive or stick and check it. Hope that works. Good luck!
I am using this card:
Samsung PRO Endurance 128GB 100MB/s (U1) MicroSDXC Memory Card with Adapter (MB-MJ128GA/AM)
And this reader:
SanDisk MobileMate USB 3.0 microSD Card Reader - SDDR-B531-GN6NN
Are you using windows to format? You can't use the native formating tool in windows as it does not do the fat32 format. It needs to be fat32 as we are running old software on the Model Y right now and exfat or other formats may not be supported. This is most likely your issue if I were to guess.
For windows you want to use guiformat downloaded here:
Ridgecrop Consultants Ltd
Make sure you pick the right drive and you don't accidently format the wrong drive. Make sure the white box under "volume label" is blank. You can keep quick format checked and leave the allocation unit size alone. After you format it, open the drive, create a new folder and name it exactly TeslaCam
Make sure there is no spaces and the T and C are capitalised. Then that is it. Don't do anything else to it or put anything else in it. Eject it from your computer and plug it into the USB A slot under the Tesla screen. Then you need to go into your options and enable sentry mode.