How much faster is the memory usually than the disk?

It's not precisely about SCSI drives, but I think that the Latency Numbers Every Programmer Should Know table could assist you in understanding the speed and the difference between different latency numbers, including storage options.

Latency Comparison Numbers (~2012)
----------------------------------
L1 cache reference                           0.5 ns
Branch mispredict                            5   ns
L2 cache reference                           7   ns                      14x L1 cache
Mutex lock/unlock                           25   ns
Main memory reference                      100   ns                      20x L2 cache, 200x L1 cache
Compress 1K bytes with Zippy             3,000   ns        3 us
Send 1K bytes over 1 Gbps network       10,000   ns       10 us
Read 4K randomly from SSD*             150,000   ns      150 us          ~1GB/sec SSD
Read 1 MB sequentially from memory     250,000   ns      250 us
Round trip within same datacenter      500,000   ns      500 us
Read 1 MB sequentially from SSD*     1,000,000   ns    1,000 us    1 ms  ~1GB/sec SSD, 4X memory
Disk seek                           10,000,000   ns   10,000 us   10 ms  20x datacenter roundtrip
Read 1 MB sequentially from disk    20,000,000   ns   20,000 us   20 ms  80x memory, 20X SSD
Send packet CA->Netherlands->CA    150,000,000   ns  150,000 us  150 ms

Here is a great visual representation that will help you to better understand the scale: https://people.eecs.berkeley.edu/~rcs/research/interactive_latency.html

Accessing the RAM is in the order of nanoseconds ( 10e-9 seconds ), while accessing data on the disk or the network is in the order of milliseconds (10e-3 seconds).

from Node.JS Design Patterns