"In VNR you can make own ECC by BCH object for new controllers." Holy cow, brainhurt!
main problem is there are no "Main Problems".
there are around, I would say, 20 common problems, but add to that the uncommon ones. if you take a flash drive you can break it down to several seperate components, all quite separate and with their own subset of potential problems, and all somewhat interchangeable:
PCB - Broken, traces broken, bad solder, bad design, discreet failure, component unsoldered, overheat... etc
Controller chip - un"cracked", uses hard to figure out methods, encrypted etc
NAND Chip - poor quality, damaged, dirty pins, no pinout, need power mod, not figured out all CE, overheated causing damage, shorted, use different system than reader (3d nand or whatever I guess), needs block cut or image cut or some other transformation
Firmware - corrupt, encrypting, dynamic xor etc, different XOR, U3, CDROM partitions, weird security..
DATA - corrupt, erased, not have file format supported,
Filesystem - corrupted, unsupported, Damaged FAT etc
Wear levelling - dynamic XOR, complex mixes like block rotate, divide, strange things like block pair 0x100000000
also might have things like 5 separate PS2251-67-5 devices with all different layout/xor/mix etc
plus you never know what is inside a drive till you pop it open