Various causes: There are very many causes, from congestive heart failure, to lymphedema, to venous insufficiency, blood clots, pregnancy, a various other causes, including serious and not-so-serious causes.
I have a Sony DVP ns55p blu-ray player with a usb port in the front. The player requires that the usb flash device (thumb drive) be formatted in FAT32in order to recognize it.
The Dell USB Ultra Slim drive still appears to be available from dell.com: Highlights. Low-profile (14.1 mm) and lightweight (less than 250 g1), the Dell External USB Ultra Slim DVD+/-RW Drive is convenient for almost any work space. plug-and-play simplicity, all you need is a USB port.