How do I setup my Dell 1550 running Linux RedHat 7.x to accept a new,
second SCSI hard drive. The drive is already in a carrier and ready
for insertion. But - what do I need to do to tell the OS about the
drive?

Congratulations on playing with SCSI; a lot of people are scared away
but it's really much easier than it looks.

If it's a newer drive, on a newer controller, there's just about
nothing to do.

The drive will automatically have an entry in /dev.  Linux isn't very
helpful with naming SCSI devices like it is with IDE devices (Anything
called /dev/hdf is the tertiary slave, but /dev/sdc could be any SCSI
ID, though it'll be the third-lowest ID)  Check your dmesg output to
be sure what your IDs are.  'dmesg |grep sd' should do it.  Find the
drive you want and remember what it is.

You will want to first partition the drive.  Use, for example, fdisk. 
fdisk /dev/sda, for example.  Check out the man page for nontrivial
commands, but if you just want to create one big partition, hit n for
new partition and create a new primary partition, accepting the
default values for start and end sectors (it will fill the drive). 
Then write the new partition table and quit (w).

Next, you will have to format the partition you have created.  Once
again, check the man page for your favorite partition type.  Redhat
7.x is happiest with ext3, a journaling filesystem built on ext2, a
longstanding linux standard filesystem.  An example command invocation
would be mkfs.ext3 -L "newSCSI" /dev/sda1

Last but not least, you'll want to mount the partition so you can
actually use it!  You can mount a whole bunch of ways, but the
simplest is "mount /dev/sda1 /mnt" which will tack your drive onto
/mnt.  You might want to make a more permanent home for it somewhere,
and add an entry in /etc/fstab for faster mounting/unmounting.


If you would like more details, just ask.  Let me know where your
drive is, where you'd like to mount it, and the like.

-Haversian

Great detail. I had started some parallel queries with knowlegable
friends and this answer was as good or better. Thanks!