Build 19613.1000 still has similar problems mounting ISO files either from Internal SATA or external USB 2.0 and USB 3.0 devices.
One particular ISO file can reproduce the problems easily.
The official MS 19041.84 ISO either one for the Slow or Fast ring.
1) Mounting the ISO from Internal SATA shows several seconds delay, but once the mount completes seems to work correctly.
2) Mounting the same ISO file from a USB 2.0 port increases the delay significantly to over 30 sec, but again once is complete the access the the Virtual DVD seems to work.
3) Mounting the same ISO file from a USB 3.0 port does not freeze like it did on build 19608, but on top of the long delay to complete the mount, the Virtual DVD disconnects and cannot see the files anymore like the Virtual DVD gets ejected on its own.