hello guys
I have a laptop toshiba satellite pro U4oo-k13 that have one hardisk and 2 partitions:
fdisk -l /dev/sda

Disk /dev/sda: 250.0 GB, 250059350016 bytes
255 heads, 63 sectors/track, 30401 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0xbcfb3865

Device Boot Start End Blocks Id System
/dev/sda1 1 192 1536000 27 Unknown
/dev/sda2 * 192 15536 123248640 7 HPFS/NTFS
/dev/sda3 15536 30402 119412736 7 HPFS/NTFS

and an external USB SDB:
16 sdb 3768 398527 408036 29868 10 0 56 28 0 12712 29896
8 17 sdb1 45 1352 1460 804 0 0 0 0 0 596 804
8 18 sdb2 2 0 4 56 0 0 0 0 0 56 56
8 21 sdb5 1299 151878 153728 8384 0 0 0 0 0 3032 8384

I wish to install bt4 beta on the sdb1 knowing that sdb5 contains data that i want, and wouldn't want to loose. plus I want the bootloader to be installed on it so i can boot to it only when I plug the external harddisk and point the boot menu to it.
now am not an experienced guy with Linux partitionning, Linux install made before where with an install application like fedora. so if there's anyone willing to help me with a step by step setup i would be very great-full.
thanks in advance