I had the same issue with 4500-X, USB file when connected to the switch would become 0 in size. Reformatting my USB with fat32 worked, solution provided in this forum is correct.
We have the same issue, only difference is that we experience this with Cisco ATA191 that we are using for faxes. These have issue where they are dropping from the network and we have to power cycle them to reestablish their connection.
I am in a similar situation, few not all access ports report link downs on a regular basis and removing/adding snmp config does not help. Our reporting process is via CiscoPrime, could it be a Prime issue?.