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there are certainly areas that a 10K rpm hard drive would offer benefits over an SSD
(...apart the obvious actual cost-per-gigabyte)

Write amplification. 

Hard drives can directly over-write a sector, but NAND-Flash SSDs cannot overwrite a page. The entire block must be erased, and then the page can be re-used. If there is other data in the block's other pages, it must be moved to a different block, before the erase.

A common block size is 512KiB, and a common page size is 4KiB. So if you write 4KiB of data, and that write needs to be done to a used block, that means at least 508 KiB of extra writes have to occur first; that's an inflation rate of 127x. You might be able to write 2x or 3x as fast as you can to your 10,000 rpm hard drive, but you may also end up writing 127x more data. If you are using your drive for small files, write amplification will hurt you in the long run.

Due to the nature of flash memory's operation, data cannot be directly overwritten as it can in a hard disk drive.

Typical block sizes include:

32 pages of 512+16 bytes each for a block size of 16 KiB
64 pages of 2,048+64 bytes each for a block size of 128 KiB
64 pages of 4,096+128 bytes each for a block size of 256 KiB
128 pages of 4,096+128 bytes each for a block size of 512 KiB

Long-Term Storage. 

Magnetic storage mediums often retain data longer when un-powered, so hard drives are better for long term archiving than NAND-Flash SSDs.

When stored offline (un-powered in shelf) in long term, the magnetic medium of HDD retains data significantly longer than flash memory used in SSDs.

Limited lifespan.

A hard drive can be re-written to until the drive breaks from wear and tear, but a NAND-Flash SSD can only reuse its pages a certain number of times. The number varies, but let's say it's 5000 times: if you reuse that page one time per day it will take over 13 years to wear out the page. This is on par with a hard drive's lifespan but that's true only without factoring in write amplification. When the number is being halved or quartered it suddenly doesn't seem so big.

MLC NAND flash is typically rated at about 5–10 k cycles for medium-capacity applications (Samsung K9G8G08U0M) and 1–3 k cycles for high-capacity applications

Power Failure. 

NAND-Flash drives don't do well with power-failures.

Bit corruption hit three devices; three had shorn writes; eight had serializability errors; one device lost one third of its data; and one SSD bricked.

Read Limits. 

You can only read data from a cell a certain number of times between erases before other cells in that block have their data damaged. To avoid this, the drive will automatically move data if the read threshold is reached. However, this contributes to write amplification. This likely won't be a problem for most home users because the read limit is very high, but for hosting websites that get high traffic it could have an impact.

If reading continually from one cell, that cell will not fail but rather one of the surrounding cells on a subsequent read. To avoid the read disturb problem the flash controller will typically count the total number of reads to a block since the last erase

(taken from here:
Sadly my projects are lost due to an HDD crash... Sad
Fundamental Collection | Vimeo channel | My blog
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