100 GB is a pretty small amount of data.

Want to minimise cost? 128 GB USB drives can be had for around ten dollars. Fifteen dollars will get you 256 GB. Stick with a cheap but reliable brand like SanDisk or PNY. It will last you years with proper care, even with regular use.

I do not really see a reason to invest in a "real" hard drive or external SSD for your use case. There isn't enough data to justify it and USB drives can take a lot of abuse and still work.

Edit: There are a lot of fake USB drives sold online where the controller has been hacked to report a higher capacity to your operating system than actually exists, for example, reporting 512 GB when there's actually only 64 GB of storage. If you try to store more than the actual capacity, your old files will be overwritten with the new ones. That's why you should be suspicious of very high capacity drives (1 TB+) sold online for low prices. I would not buy any USB drive online that claims to have a capacity greater than 512 GB.

I'd like to second the large flash drive idea. You can get a couple of them and a case to keep them in for less than $50 total and have multiple layers of redundancy

SSDs lose charge over time. Within a year, bitflips might occur. I recommend going with an HDD with a shockproof case.

This ineo t2580 looks good but I can't seem to find the maximum capacity. Try not to use HDDs over 12TB and you should be fine.

You say that data is critical for your business, have you considered a cloud backup as well? AWS has very cheap storage options depending on the speed the file needs to be available at. Also I’d definitely not go with flash memory, bit flips can be a real danger. I’d go with a 1 TB HDD in an enclosure, additionally to a cloud backup

As NateNate60 mentioned: USB Flash. I second this as a cost effective alternative to anything else. Corsair Survivor, Sandisk Exteme Pro and Kingston DataTraveler Flash drives to 256GB are cheaper than anything else and just as reliable.

Should you want to go the SSD route, the Corsair MX500 drives purchased with any external esata or usb chassis is the most reliable option for the price.

It depends how critical the data is.

SSDs and flash are great for speed, but as other comments have mentioned, they can eventually lose data more than other mediums. However this reportedly isn't likely to happen over the normal rated timespan of the devices, unless there is a critical defect.

Magnetic storage will likely last longer, and as it is a much older tech, is less likely to have firmware bugs and other problematic surprises. Plus, as you can see on https://diskprices.com/, the cheapest medium per TB remains magnetic storage.

Then there are tapes. The drives sure cost a hefty sum, but if you have loads of data to backup, this is likely the cheapest option.

Finally, optical. Optical is great in the sense that is is physically a ROM, so data cannot get compromised by mishandling or other staff mistakes; but it still can have issues with the reflective layers peeling away from disks.

So, in the end, I would personally not recommend using SSDs for data backups, out of precaution. Sure, SSDs will likely retain all data just fine for years to come, but I want to be able to store data for as long as possible, with the peace of mind that only magnetic storage will afford me. Plus, if your data is worth backing up, it is worth whatever extra price or effort you will have to do with.

As for the other options, well, they all have their use case, but I don't see much advantage for them in the general use cases. Just make backup copies of your data on magnetic drives, in a few physically different locations, with proper access control.

If you want a 2.5" form factor and plan on moving it in any sort of way, SSD is the way to go.