SXS UDF cards work and don't

Hi, wondering if anybody has seen something like this:

Two clean installs of El Capitan, FCPX 10.2.2 and other software. Install 1 is on a 2012 MacBook Pro 15" 8GB Ram, Install 2 is on a late 2012 iMac 27" 32GB Ram. I installed the latest version of the SXS UDF driver on both. The Device driver was not installed as I use the Sony USB Card reader SBAC US30 with both computers.

On the MacBook Pro, I have no problem viewing xdcam 422 HD 50i 50Mb/s in Sony Catalyst Browse or in FCPX 10.2.2, both quite snappy with the card material.

On the iMac, I can view the same material in Sony Catalyst Browse though it is a bit sluggish, and in FCPX 10.2.2. the card mounts in the import window but all clips are black and response grinds to a halt. The "timer" icon next to the card in the upper left seems stuck at "2 o clock"

No problem on both computers when using non udf formatted cards (XDCAM HD 422 50i 35 Mb/s)

I am aware of the black frames problems with Panasonic material and I can work around this in a similar way, using edit Ready for instance. But this should be working in both cases and it drives me a tad nuts that it does not.