So I've started to dig into storage fundamentals and found that in order to calculate the IOPS of a HDD you need to know the internal transfer rate of the drive (time it takes data to move from the platters to internal disk's cache).

I went on newegg and even a few vendor sites and could not find this info published for any HDDs. Is it sometimes called something else? Take this link to a seagate HDD for instance. Nowhere do I see "internal transfer rate", but I do see something called "Sustained Data Rate OD"- is that the same thing?

Just so you know where I'm getting this info (Book: "Information Storage and Management Storing, Managing..."):

Consider an example with the following specifications provided for a disk:

The average seek time is 5 ms in a random I/O environment; therefore, 
T = 5 ms.

Disk rotation speed of 15,000 revolutions per minute or 250 revolutions per 
second — from which rotational latency (L) can be determined, which is 
one-half of the time taken for a full rotation or L = (0.5/250 rps expressed 
in ms).

40 MB/s internal data transfer rate, from which the internal transfer time 
(X) is derived based on the block size of the I/O — for example, an I/O 
with a block size of 32 KB; therefore X = 32 KB/40 MB.

Consequently, the time taken by the I/O controller to serve an I/O of block 
size 32 KB is (TS) = 5 ms + (0.5/250) + 32 KB/40 MB = 7.8 ms.

Therefore, the maximum number of I/Os serviced per second or IOPS is 
(1/TS) = 1/(7.8 × 10^-3) = 128 IOPS.