What type of USB CD-ROM are you using? We have a TEAC CD-210PU that works for us on a PXI-8186.
Also, do you have the latest version of the BIOS? The latest version is 1.2 and you can get it from our ftp site at:
ftp://ftp.ni.com/support/pxi/pxi-8180/PXI-8186/BIOS/
Hope this helps,
Jack