With a SCSI controller installed, when the machine is powered up or reset the SCSI controller will scan the SCSI bus looking for attached devices before booting the operating system.
If your SCSI card is correctly installed, and your Jaz drive is attached and powered on, you should see the drive listed as something like "Iomega Jaz 1GB" or "Iomega Jaz 2GB" during this time.
If the drive doesn't show up, there's no sense booting Linux. Power down the PC and Jaz drive, and check everything again. In particular, unplug the cables and make sure none of the the pins are bent, then replug them and ensure they are completely seated against the connector. If you have multiple SCSI devices, make sure they all have different ID numbers, and that the last device in the SCSI chain is terminated.
When the drive shows up in the power-on test, you're half way home.
When Linux boots the SCSI driver should display information about your SCSI adapter and what devices are attached to the SCSI bus.
Boot messages will vary depending on your driver and adapter, and are logged to the /var/log/messages (or /var/adm/messages) file as well as appearing on the screen during boot. You can also 'replay' the messages since your last boot from the command prompt with the dmesg command.
Here's the kernel booting output from a 2.0.36 kernel with an Adaptec 2940 controller (using the aic7xxx driver):
(scsi0) <Adaptec AHA-2940A Ultra SCSI host adapter> found at PCI 14/0
(scsi0) Narrow Channel, SCSI ID=7, 3/255 SCBs
(scsi0) Warning - detected auto-termination
(scsi0) Please verify driver detected settings are correct.
(scsi0) If not, then please properly set the device termination
(scsi0) in the Adaptec SCSI BIOS by hitting CTRL-A when prompted
(scsi0) during machine bootup.
(scsi0) Cables present (Int-50 NO, Ext-50 YES)
(scsi0) Downloading sequencer code... 419 instructions downloaded
scsi0 : Adaptec AHA274x/284x/294x (EISA/VLB/PCI-Fast SCSI) 5.1.2/3.2.4
<Adaptec AHA-2940A Ultra SCSI host adapter>
scsi : 1 host.
Vendor: iomega Model: jaz 2GB Rev: E.17
Type: Direct-Access ANSI SCSI revision: 02
Detected scsi removable disk sda at scsi0, channel 0, id 4, lun 0
scsi : detected 1 SCSI disks total.
(scsi0:0:4:0) Synchronous at 10.0 Mbyte/sec, offset 15.
sda : READ CAPACITY failed.
sda : status = 1, message = 00, host = 0, driver = 28
sda : extended sense code = 2
sda : block size assumed to be 512 bytes, disk size 1GB.
If something resembling these lines doesn't appear, then your kernel is probably not configured correctly (or you picked the wrong pre-built kernel).
If the "scsi0 :" line does not appear, then you have not configured your driver correctly. Some drivers will give you a hint about what is wrong. If a drive name is not assigned, you probably forgot to include SCSI disk support when you built the kernel.
Note the READ CAPACITY failed. Most SCSI drivers (like this one) will detect that the drive is a removable media type, and not get all flustered when it can't read the partition table. However, some SCSI cards aren't as graceful. If your PC hangs during boot, try booting with a cartridge in the drive.
Check the README files in /usr/src/linux/drivers/scsi and the Kernel HOWTO for other debugging hints.
In the log output in the previous section note the raw SCSI device name, which in this case is "sda", the full name being /dev/sda. If the Jaz drive is the only SCSI disk on your system, it will typically be /dev/sda. If your primary hard drive is SCSI, or you have a SCSI CD-ROM drive, it could be sdb, sdc, etc.
Whatever the drive name is, you will need this name to access the drive, as explained in later sections.