am configured usb as both USB ACM and USB MASS STORAGE simultaniously?

[Macherla Swetha]

When the USB stack and your local Filesystem code both try to control the flash chip simultaneously, the system will crash or trigger a HardFault, causing the restart.

[Maria Gilje]

Hello, 

Without learning a bit more about what the hardfault is about, it it hard to say what the fix for this is. However, this DevZone ticket from last year describes an assert which may be relevant for your issue:  USB driver : setting 'high-speed' causes assert to fail for MSC subsys as NRF_USBD_COMMON_EPSIZE is fixed at 64! 

If it is not relevant, please refer to the documentation on USB composite device for the Thingy:53 and the CONFIG_USB_COMPOSITE_DEVICE Kconfig symbol. And if you want support for your specific hardfault, please share the log with us. We advise you to use the Insert->Code option when sharing logs and code snippets. 

Best regards,

Maria