These are the general paths you might find:
HDD1:\ <--- This is your internal harddrive, you store profiles, XBLA and DLC etc. on this...
HDDX:\ <--- This is the Xbox Compatibility partition of your internal harddrive
Flash:\ <--- This is the NAND filesystem (DO NOT TOUCH IT UNLESS YOU KNOW WHAT YOU ARE DOING!!!)
Game:\ <--- This is a virtual path to whatever game you're currently running, leave it be, it's better to use the other paths...
USB#:\ <--- This is your USB drive(s) # == a number
USBMU#:\ <--- This is your USB drive(s) configured by the dashboard... # == a number...
USBMU#Cache:\ <--- this is the cache part of USBMU#... it's used only for TitleUpdates, no need to worry about this one...
OnBoardMU:\ <--- This is the built-in memory unit of a trinity, same as USBMU except it's built into the motherboard...
SFCMU:\ <--- Same as above, except for Jasper's
MMCMU:\ <--- Same as above, except for MMC Corona's
MU#:\ <--- This one is only available on Phat Xbox 360's and it's the small Memory Unit you can attatch to the front ports...
Some of these might have different names depending on the homebrew you're using, but they generally have a similar name...