i am not online much so ill try to answer possible solution on speculations of what you meant.
A)you converted to dex and updated to 4.21dex
1)have you put a game on one of the partitions via ps3gen?
2)have you done it with the correct procedure?
3)if you didnt put it on slot0 but on another slot(partition) use multiman DEX to switch to the correct slot(ps3 must be connected to a router via wire and set up to be connected to that router via ps3 settings)
4)if the drive doesnt show up in the ps3gen so you can write to it run ps3gen as administrator THIS IS OBLIGATORY.
B)you accidently updated to OFW or anything else so unfortunately if you dont have a HW flasher there is nothing you can do(yet).
C) if you solve your problem i recomend using aldos hdd partiton tool to make more partitions and resize them depending on the game you put on each slot.Also remember that multiman DEX currently supports up to 10 slots if you make more the games from slot11 and up will need a computer with PRODG etc to be loaded(i wont write the whole procedure search it).
Just my 2 cents