A couple (or more) years ago I was planning to assemble an
external DVD burner and, after some failures with the
ALI-based Bytecc enclosures, saw a recommendation for the
Plumax case based on the PL-3507.
It's labelled PM-525C2-PPSO on the box; it's a silver combo
USB2/Fireware interface. I'd never used it, and it's been
on the shelf until recently when I decided to try it with
an working Pioneer A07 (DVR-107D) internal which has
been replaced by a faster drive.

There are no problems with aborted burns or data corruption
(unlike the Bytecc) but transfer rates are very slow;
pretty much topping out at 6X for reads and 3X for writes.
This is with 8X media that the drive has successfully
written at 8X when operating as an internal drive, so media
is not the issue. Nor is the host computer, as I've gotten
the same consistent results on 3 different machines with
verified USB2 ports using different controller chipsets.

What's going on? In some of the multiple 'external
enclosures' threads out there, I've read references that
indicate the PL-3507 boxes usually need firmware updates,
but I didn't see anything explicitly addressing my issue
in the changelogs. Is it worth a flash? Which version?
Thanks for any guidance.

Well, I updated the PL3507 bridge firmware to the 4.20.2006 code at the Prolific support website, and transfers seem more in line with what I'd expect.