@Red-M Yes, thank you for detailing the factors for others here. However, I really do understand these factors and have been using USB MSC on mounted SDcards with STM32 for 1000's hours with no corruption. The repeated assertion here that using USB MSC will sooner or later corrupt the file system is not accurate.

What will corrupt your flash is power disruption, or bad code, or bad user behavior. But fundamentally, an embedded system that is writing to its system flash is fundamentally less robust regardless if you have USB or not. Note that I'm somewhat centric to SDcard usage because I'm developing applications that are too large for typical MCU internal flash. The latest ESP32's are interesting but have storage performance issues. SD cards have an embedded ARM controller and do their own wear leveling now. If they are not powered down properly and are crashed during a write, their wear leveling will corrupt the entire file system. So, the recommendation is to use Industrial Grade SDcards which have more support for avoiding such corruption.

As far as making USB MSC safe to use, simply consider the following and you'll be fine...

User apps should not be randomly writing to "system" flash - this is not "best practice" and user design problem
There is no need for access arbitration. Dynamic automatic arbitration (locking) would be cool but isn't essential
Host PC write caching should be turned OFF, that's a user dev environment problem not MicroPython's
Reboot MicroPython after writing to flash from Host PC. OK to write during normal execution, but need to reboot to use
If apps such as FTP are active, don't write to the flash from PC until FTP disconnects. Be a responsible developer
Never power off a target while flash is being written do (obviously)
Never remove an SDcard while system is running

I hope I'm not just in a minority who is writing larger/more complex MicroPython applications (500K to 1MB source code)? Unfortunately, I have seen no alternative development path that compares to the efficiency and cycle time of using USB MSC. It really works really well. If unavoidable flash corruption exists, with these large projects and heavy USB MSC use, then file system corruption would be a problem for me. It isn't. What is a problem is a lack of any good alternative to USB MSC.

Thonny and rshell are incapable or supporting these projects effectively. If there is anything that compares to the performance and utility of USB MSC, please let us know here.

Uh oh!

ESP32 S2/S3 USB MSC support? #8426

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