3 would be support for an external USB floppy Disk.
4 would be support for network boot (diskless workstation)
5 is support for your external USB hard-drive (this is option you will use to boot into Linux)
6 is support for an external USB CD/DVD drive
I'm not sure what you're asking regarding the various drive letters but Linux doesn't use the same lettering scheme as Windows. Fedora uses sda, sdb, sdc and so forth. Normally, your first physical internal hard-drive is designated sda, with partition numbers following. For example, sda1 is the first partition on the first physical drive on your system. sdb3 would be the 3rd partition on the 2nd physical drive.
When I plug in a USB drive into my PC, it assigns it
sdb because the USB drive is the 2nd physical drive attached to my computer.
My USB drive only has one partition so Linux sees it this way..
Disk /dev/sdb: 1031 MB, 1031798784 bytes
255 heads, 63 sectors/track, 125 cylinders, total 2015232 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x1031223e
Device Boot Start End Blocks Id System
/dev/sdb1 * 63 2008124 1004031 6 FAT16
If I want to install Linux to my USB drive, I would install it to
sdb, writing the bootloader to the MBR on
sdb. I could then boot to Linux using the dynamic boot menu.
This is the approach you need to take as well. You can determine how Linux sees your USB drive by plugging it in and booting with the Fedora 17 live CD. Open a terminal in F17 and enter the following at the command prompt.
fdisk -l
You can post the output back to this thread and we can take a loot to see how Linux sees your drive layout.