One question, do you have to partition the storage device to be able to store, play music files in addition to storing saved dash cam, Sentry mode recordings?
Pretty sure if only one partition is presented to the car and that partition has the TeslaCam folder on it, then you cannot access that for anything else. Whether that is a separate USB stick with one partition or one of multiple partitions on a single stick probably should not make a difference, but I have not used multiple partition devices in my MY so I cannot verify that claim. I use 2 separate USB sticks, each one only has one partition, KISS principle in effect. The drive that is used for sentry is not seen when selecting a USB drive for music, I assume it is locked when in use for sentry so probably not usable for music. At least that is what I see with my config.
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One of the problems I have with using multiple partition USB devices for sentry is that you should be using high endurance drives since sentry mode is always writing video, regardless if an event is occurring or not, just so you have the 10 minutes of video prior to the event. If an event does occur, then it preserves those already written files so they do not get overwritten like they would normally when they are not needed. Normal sticks will wear out and fail quicker than a high endurance stick from all this writing. Also, a smaller drive will die quicker than a larger one, assuming wear levelling is in use, since less sectors to spread the load across. So chewing up a portion of that high endurance drive for static files like music does not make a lot of sense to me.
Additionally, removing that drive to update its music content runs the risk of not having a drive for sentry mode saving should something happen while it is removed, or if you forget to put it back in, or if you corrupt it somehow, or it does not auto detect and mount for sentry mode when you re-insert it. Tesla made a software update at some point to store 1 sentry event in the MY internal storage, if you want to rely on that and hope that one capture has everything you need. Otherwise you should always have that stick installed to capture it all.
The price for a dedicated high endurance drive for sentry mode that stays in the MY 24x7 is well worth it in my opinion. Should something ever happen and there is no video evidence, you will most likely end up with an insurance claim and/or paying out of pocket to get it fixed, which will in all likelihood cost a whole lot more than that dedicated drive.
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