I just posted this on the iMac forum. This is probably a better place for it. I don't really use DVD's much anymore, and my 2017 iMac of course does not have an internal DVD drive. But I had to do a training thing from work and they gave me a DVD. I have two external portable USB 2.0 DVD drives, an older Iomega and a new LG. What's odd is that the DVD they gave me will play just fine on a "regular," stand-alone DVD player. BUT neither my 2017 iMac using either external DVD drive nor my 2009 MacBook using either its internal DVD drive or either of the external DVD drives will recognize it, and after a few seconds or sometimes a few minutes both machines will start to freeze up, first Finder, eventually other apps and then all that will work is the mouse, requiring a hard reboot. I have tested other DVD's and they work fine with all the drives and both computers. And I have used DVD's from work in my 2007 iMac in the past with no problem (that machine is elsewhere now). What's odd to me are two things: obviously that the DVD works in a DVD player but not either DVD drive attached to either Mac, AND ESPECIALLY that this DVD seems to cast a spell over the Macs, which usually handle errant programs or processes very well. I can watch the DVD on the stand-alone player, and this seems to be a one-DVD problem, so there is no crisis here. But it is a curiosity to be solved. Any ideas would be appreciated.