On a Mac where drives mount on the desktop when connected, selecting single drive gives me one mounted pogoplug drive on my desktop instead of 3 (one for each of my 2 hard drives and the other for item shared with me). Not sure what it does on a PC. Maybe nothing and that's why it's grayed out.

I just tried the windows software in Parallels running XP. The P: directory that appears in my computer does contain all my drives and the the single drive option is grayed out. This may be a Mac only feature to choose between having each drive mount separately or together in one group. The only way to know for sure I guess would be to ask Tech Support.

The Single drive mode is actually the only supported mode in the Windows drive app (the greyed-out checkbox actually should show it as being checked - we'll fix this for the next release). In the Mac version, if you have multiple hard drives (or multiple Pogoplugs) you can decide whether each hard drive appears as a separate drive (Single drive unchecked), or as a single virtual drive with multiple folders representing the actual drives (Single drive checked - the way it works in Windows). We provided this for the Mac version because Macs handle lots of hard drives well, and also because the OS likes to write information at the root level - which we can't do with a virtual drive. Would love to hear if you want this feature available for Windows.