Sony says ( http://pro.sony.com/bbsc/ssr/product-PCMD50/
) PCM-D50 is compatible with the following Memory Stick models:
Memory Stick Pro-HG Duo MS-EX4G (4GB), MS-EX2G (2GB) , MS-EX1G (1GB)
Memory Stick Pro Duo (High Speed) MSX-M2GNU (2GB), MSXM1GNU (1GB)
There are two problems using the above products. First, the maximum capacity available is 4 GB. Second, these are the most expensive memory sticks Sony makes.
in 2008, Sony released a new version of the less expensive Memory Stick Pro Duo called the "Mark II." The Mark II cards are available in capacities up to 16 GB, and they are less expensive than the Pro Duo (High Speed) and Pro-HG memory sticks.
The Mark II was designed for use in Sony's video cameras. The latest Sony video cameras use AVCHD format (a version of the H.264 standard for video compression). Recording mid to high quality HD video, the cameras generate a data stream of 6 Mb per second to 13 Mb per second. The Mark II cards are rated for a 32 Mb per second continuous write speed, so they can keep up with AVCHD video.
Read the specs for the Pro Duo Mark II herehttp://www.sonystyle.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10551&storeId=10151&langId=-1&productId=8198552921665359950
If you were recording stereo audio on your D50 at a rate of 24/96, you'd create a data stream of 4.6 Mb per second. That's considerably less data than the AVCHD process creates. So even though Sony does not recommend the Pro Duo Mark II cards for the D50, I think they will work fine.