One of the major problems I have when doing a project with retro hardware has become mass storage. There are many options available, ranging from the original drive to Compact Flash to IDE adapters and their close cousin SD card to IDE (and SCSI) adapters, however, they all have a couple of flaws.
With original hard drives, well, they're old - they have a limited life span, and if you're looking at something that might have been manufactured decades ago and are well past their expected life span. They're also generally slower than any flash-based option and they range from loud to really loud to Quantum Bigfoot loud.
Flash memory to IDE adapters is a combination of somewhat expensive, unreliable, hard to find media for, and not designed to be used as an OS drive, especially with an OS that doesn't have any concessions made for media that's not designed to be written to at random with whatever and whenever it feels like it.
I've had miserable luck with SD to IDE converters in particular - they're way flakier in terms of suddenly not being able to be accessed, and in some OSes (OS/2 in particular) they have constant data corruption and data loss issues which have made a few projects I've worked on impossible - if you lose data every week or so, you spend all your time re-doing the last bit of work you did and never actually make progress on what you're meaning to do.
Worse, Compact Flash cards - which have very good compatibility - as they should since they're just essentially an IDE drive - are no longer made in small capacities which means you're buying random used cards of unknown quality and vintage off eBay or hoping whatever you get from AliExpress is legitimately what it claims, and even if a larger card (32gb+) isn't an issue, they're extremely expensive compared to other types of flash, and substantially more costly than my current preferred solution.
One option that is gets missed because it also has some drawbacks, though I've found them to be easy to handle, is using a SATA to IDE adapter, and a modern small SSD.
The primary issue is that old hardware does not understand how to deal with drives over common capacities available at the time, and even more modern Pentium 2 or 3 era hardware may have issues with drives over 8GB. You can't just drop the SSD onto an adapter, plug it in, and have it work.
In some cases, the BIOS will have the ability to restrict the size to 8GB, either as a direct option, or you can configure the drive manually to stay under the limit, though I tend to prefer to utilize a more "modern" ATA-133 adapter card - most of these have a jumper or card BIOS option to restrict the visible maximum size of the drive to 8GB, and they're almost certainly substantially faster than what's onboard already which is nice when you're talking about using a modern SSD.
The cards themselves are easy to find; I have a local Goodwill that will have a box of these at any given time for $1 each, but they're not too expensive on eBay or other online marketplaces either.
SSDs are a little more expensive than a SD card, but places like Microcenter sell their Inland brand 120gb SSDs for $20 (or less, when on sale), and those are the specific drives I use. They're relatively slow for SSDs, do not have the most impressive random read and write performance, but are so far outside of what a retro computer could expect that it doesn't matter for this application.
Setup, once you've determined how you're going to restrict usable capacity, is just like any other drive - it shows up as an 8GB drive to whatever OS you're using it with, and I've successfully used this with OS/2 Warp 4.5, MS-DOS 6.22, and Windows 95 OSR2.5.
It's not a perfect solution - the SATA to IDE adapters are not universally compatible with EVERY ATA controller, they're a little expensive, and they require additional hardware eating a slot in your old computer, but it does result in fast, cheap storage that's easily obtainable, doesn't exhibit the issues I was having with SD cards, and doesn't rely on a type of media that's basically no longer made as it's been relegated to complete niche solutions.