What is the largest SSD that an iMac 21.5 (Late 2015) can use

AFAIK currently there is no capacity limitation.

You did not indicate you were using a Fusion drive. I would recommend installing a 1-2 TB SSD and then using your internal HD for additional storage.

No drawbacks. Simply install the new SSD which will unfuse the Fusion drive on it's own. Then install Mac OS on the new SSD, and migrate your data from the HD to the new SSD. Once done, reformat the internal drive after you have tested that it is working okay and your data is available. Once it is tested and working okay, then you are good to go on the HD format.

"What is the life expectancy of a SSD compared to a rotary HD these days if your constantly using it?"

For SSD, as stated, the lifetime is based on the number of erase/write

cycles. Current state of the technology puts that number in the 2-3K

range and somewhat better for premium "enterprise rated" SSDs.

So, if you wrote to a single byte every day, it could be expected to

last over 5 years. Also keep in mind, these numbers are "bell curve"

based so some would be longer, some shorter.

The SSDs have multiple ways to extend the overall lifetime by "wear leveling"

and over provisioning which just means there is more storage than actually stated.

When it is all said and done, the computer itself will likely lose its usefulness

before the SSD "wears out".

As far as comparing the two, because of the mechanical complexity of an HDD,

it is likely to fail before an SSD. However, both can fail on day one or last far

beyond the usefulness of the computer or anywhere in between.

What wears out first on ANY machine is the moving parts as a general rule of thumb. As SSD have zero moving parts in theory they should last significantly longer than a HD.

What is the life expectancy of a SSD compared to a rotary HD these days if your constantly using it?

...one day longer than the warranty.

higherterrain

Got the present SSD going south on  on the iMac. 

Just curious what is the problem with the current SSD and how do you know it is bad? If the SSD is bad, would you mind posting a screenshot of the "Health Indicators" from DriveDX?

higherterrain

What is the life expectancy of a SSD compared to a rotary HD these days if your 
constantly using it?

What wears out an SSD is the number of writes to it. Every write to an SSD necessitates an erase. This combination of write & erase is what kills an SSD eventually. Each block can only handle so many of these events (called P/E cycles). Keeping the SSD nearly full can accelerate the process and impede functionality of the drive in addition to the OS issues it would normally create. Find out the specs on the SSD to compute how many years it will last by writing "X" amount of data per day. Most should last years under normal use even writing 20GBs per day. Check out the SSD endurance test which ran SSDs 24/7 a few years ago.

Of course some SSD's just end up dying prematurely without any warning usually due to some controller issue.