Hk.t.rt2841p631: Firmware ((free))

Capacity: 8GB or 16GB maximum. Many RT2841P chips do not recognize 32GB or 64GB cards in bootloader mode.
Format: FAT32 (not exFAT or NTFS). Use a tool like guiformat if Windows won't let you format large cards to FAT32.
Speed: Class 10 or UHS-1. Slow cards cause firmware write errors.

Stay safe, and always backup your current firmware before updating.

Official firmware download from vendor website (preferred). Verify checksums and signatures.
Local dump via serial or JTAG (requires hardware access and expertise).
Extraction from device using firmware update package (.bin, .trx, .img). Use binwalk and extraction tools.
Legal: only analyze firmware you own or have explicit permission to test. Abide by laws and terms of service.