Same problem here (Sony Nwz-E445)
When plugged to Mac OS X it does not appear anywhere on the desktop or in Disk Utility (I can see the device in "about this Mac/system report/USB, though) but it charges ...
To Windows Vista, it appears listed as a portable device in the device manager, but there's no way to access anything or format it. If I try to run the firmware update, it says that there is no connected device. Tried to change it from MTP to USB mass storage to no avail.
With Linux Mint 17, it just says "Can't mount the device" or something, then it appears in the explorer and I have the message, when I double click on the icon "Please insert a disk"
I tried to reset when plugged to the computer but it didn't work.
Any suggestion is more than welcome :)
Pierre
November 2015