I have a 512GB SSD and a 4TB Western Digital Black. I have 212GB free on the SSD and 3.6TB free on the HDD. I thought I would need a lot of storage for my stuff, but no. Also, everyone I know who aren't gamers uses only 250GB of storage. Do we really need over 1TB?
It seems that every year or so, the makers of electro-mechanical drives produce new models with greater capacity. In fact, this also generally applies to include SSDs.
I've always been a bit reticent to increase my available storage with devices of higher capacity when the storage I have seems ample.
On the other hand, I personally think one is better off with a storage device filled to about half capacity than with a device at 70% or 80%. I remember the old days when you could fill up a disk to the point where Windows couldn't function properly. Perhaps it couldn't find enough space for a page file -- any number of things.
I'm also posting here in threads about my WHS 2011 home server. It uses a drive pool of four disks -- 1TB each. It's more capacity than I need at the moment. But I can envision a situation where I might fill the pool with DVR archives or audio recordings as the collection grows. I just think I have a long way to go with this.
The problem with higher-capacity storage is that one must also make provision for backup devices sufficient for the files or categories of files determined to be of higher priority for backup. I currently am using about 1.3TB out of my 3.6TB drive pool on the server. To back up my most important files, I need a 500GB drive -- and as expressed in another thread I started here -- I currently have to back up those files and folders effectively twice, since they're duplicated folders in the pool and there's not an easy way to determine which disks hold a single copy of the entire collection of those files. Until Cove-Cube -- developers of StableBit Drive Pool -- can fix their program to allow other backup programs like Acronis to access the pool successfully, the individual pool members need to be backed up.
But that's the essence of the problem. You want enough storage space for current and future needs. You also want some sort of internal or external device which is sufficient for backup. How will you back up? You might take disk images; you might clone a hard disk to another hard disk; you might simply do a "COPY" or "XCOPY" of files from folders on one disk to folders on the backup disk.
The problem doesn't change when the focus is HDDs or SSDs. Your largest-capacity option is an HDD. But Samsung EVO SSDs come in a 1TB model. For me, I could "almost" get by with a 240 or 256GB boot drive, but that's cutting it close. My ISRT-leveraged 600GB HDD will probably have 50% unused capacity two years from now.
Unless it's a matter of speed (SSD) versus capacity (HDD), HDD prices are so low as to make the issue of "buying too much" sort of trivial. And there are at least a couple ways to leverage SSD speed while much of the storage location is on the HDD. With current SSD prices, users need to decide for themselves whether they really need a 500GB or 1TB SSD for anywhere between $300 and $500. Those prices are coming down. But so are the prices on 250GB models. I noticed just today that you can get a 240GB SSD for maybe $120 -- as long as it's not a Samsung or Intel model.