You did well, I had the same yesterday. I then added 1627 with bdm as title in fmcb, on a paralel OPL (kept the 1629 settings as bdm3 title) to see how it would work and it did the same. However, after doing it a few times and resetting the ps2 several times the save option did go through somehow .
It ended up saving anyway.
You can keep as many opl versions in the same folder on mc0: as you like.
I keep having the issue that 1629 can see but does not play USB. I'm using 1345 for that.
This is a fat PS2. I've got a TON of games on the HDD and I also want to use my MX4SIO so I need to be able to save. Hopefully someone can chime in here on what else I need to do in order for this to work.