A Xbox HD partition should have C, D (DVD-ROM drive), E, F, G (if enabled in your dash), X, Y, and Z partitions. G is optional, some dashes like Avalaunch have the option to enable the G drive. On a modchipped Xbox, typically F is the main partition, where most movies, and music are stored. C is usually the boot partition for a chipped Xbox, i.e. where the dash files are for your default dash, as well as the MS Dash. So you don't want to mess with C (changing your dash for example) without knowing what you are doing. E usually is the home for your applications, emulators, save games and such. On a softmodded Xbox, E will usually be the main partition, as opposed to F on a modchipped Xbox. The Y and Z partitions contain system files and cached files for XBMC and stuff. Typically you really don't want to mess with them.
Does your Xbox have a stock Hard Drive? The stock HD is only 8GB's. If you only have a stock HD, then streaming music, videos over a network with XBMC is a better idea than storing stuff on the HD. If you have an retail HD, then it's a good idea to FTP files that are taking up a lot of space to your PC and burn them to DVD with Nero selecting the DVD-ROM Iso option. You kinda have to space out the files, because a single layer disc holds about 4.37GB's. Keep that in mind when FTPing huge groups of files. Here's a guide for streaming media files from a Windows PC to Xbox Media Center.
Windows File Sharing (SMB/SAMBA/CIFS) - XBox Media Center