Atleast one less problem.
Erm,... I forgot,... Have you mentioned yet, that you activated the USB-Device (or any other device) in OPL's settings?
Try to set it to both, 'Manual' and 'Auto'.
Not sure about that,... I guess, it has problems reading or identifying your disc.
Yepp, so this is o.k., since it is not an RW (=
Re
Writeable), but only R (=
Recordable).

But it is of the
PLUS-Type, whereas I already told you that if you have a
PS2-Model prior to SCPH-500xx, it
will not recognize it if you dont
'booktype' it.
Some additional info would be good:
-PS2-Model (SCPH)
-DVD-Writer-Model and if it is updated to latest Firmware.
ESR.ELF (in capital letters and yes, that makes a difference on PS2... Not all 'operating-systems' ignore that like Windows...)!
...and normally it is installed to the BOOT-Folder, when you used the Nooby-Package for your FMCB-Install...
...or have you used the plain FMCB and now added your apps on your own???
I try it...
Let's hope there is a solution to your problem.