Nextion’s microSD slot is primarily used to upload a TFT project file.
– The Intelligent Series has added capabilities, see Forum Announcements

Not all microSD cards are made for use with embedded devices.
– especially newer microSD cards made for cameras, etc.
Class 10 HC 8GB to 32GB cards have had good success. Others too.

– microSD card formatted as FAT32 under Windows
– microSD card is less than 32 GB
– only 1 TFT project file exists on the microSD card
– Nextion device is powered off  before inserting microSD card
– insert the microSD card containing TFT project file
– power on Nextion with recommended clean power as per Datasheets
– Nextion device is undisturbed while uploading
– after upload “successed” indication power off Nextion
– after Nextion device is powered off  then remove microSD card
– power on Nextion with recommended clean power as per Datasheets
– if new version of Nextion Editor, allow time for firmware to update
– wait for uploaded project to begin running on Nextion device

Data Error can occur with newer Nextion devices needing v1.63.3 Nextion Editor
– if receiving Data Error, you must use v1.63.3 or above (card noting this included in box)

If  microSD upload is unsuccessful
– Reformat the microSD card –  ensuring FAT32 under Windows, and try.
– use a different microSD card (HC, Class 10, under 32GB), and try.
– compile a blank HMI project and try again.
– microSD card is not the only method to upload project – try via USB to TTL.

Using the above steps, the Nextion device will always run firmware upload
when an inserted microSD is detected at power on (power off before insert/remove)