Live USB F12; processes in "Disk sleep", unusable

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http://forums.fedoraforum.org – While running an F12 Live USB from a flash stick, processes performing heavy-ish disk I/O go into a "disk sleep" state (STAT column is "D" in `ps`) - this causes the system to grind to a halt. This happens particularly when using yum to install a package or two - several processes (including yum, usb-storage, and a few k* kernel procs) enter this state, which slows down everything. (trying a `yum update` actually made the system completely unresponsive; I walked away for three hours and only 150 packages had actually been downloaded). I tried disabling power management on the disk with `echo on > /sys/bus/usb/<deviceid>/power/level' per the kernel docs at /Documentation/usb/power-management.txt, but that did nothing. Is this an unavoidable by-product of running off a USB flash drive? I thought that flash was supposed to be fast; I cannot imagine it takes so long for I/O that processes have to block so much that it takes the whole system down.... Could it be my device? It's an almost-new 16gb sandisk cruzer, that worked previously. Any next-steps for diagnosis would be much appreciated. Thanks for your help. (HowTos)