Have you considered using them for RAID?

https://en.wikipedia.org/wiki/RAID

You’d still need the slots and some raid modes are limited by the size of the smallest disk. Drives being as cheap as they are, it’s often not economically viable to retain the old drives.

If you use software-raid you can use striping.

Basically, divide up each disk into 1TB partitions, then RAID the 1TB partitions together. Synology uses this for their Hybrid RAID. Obviously care needs to be taken to not confuse partitions but this lets you use disks of differing sizes.

Unraid also offers RAID with differing sizes for disks, it has quite different failure modes though but it's been fairly reliable for me.

Affordable RAID controllers end up being more failure prone than the drives attached to them.

You don't need hardware raid controllers. (Unless your really need them for some reason) Software RAID is perfectly fine for most use cases.

Affordable disk shelves - something like the Netapp DS4243 - on the other hand tend to be reliable and give plenty of space for SAS and SATA drives. I bought a fully populated shelf (4 x powersupply (of which I'm only using 2, the rest are disconnected and meant as spares), 2 controllers and 24 15K SAS drives (mostly HGST, a few Seagate drives) for around €400. I'm currently running it with a number of those 15K drives but will eventually replace some of them with slower but bigger SAS or SATA drives, not so much to get more space but to save power. For now the thing runs fine, the error logs on the drives are mostly clean (2 drives with a few errors, mostly likely due to power glitches as they occurred a long time ago and have not recurred). If you want lots of space for drives with redundant power and cooling these shelves are a good value since there are plenty of them on the market which keeps prices down.

I'm using a combination of striping, RAID and mirroring depending on the content, using lvm and mdadm instead of ZFS for the increased flexibility. The server itself contains a HP R410i RAID card which drives 8 internal 2.5" SAS drives, the thing could be expanded for not that much money to a 16 x 2.5" array but I see no reason to do so given the higher prices for 2.5" drives and the abundance of capacity offered by the DS4243.

An mdadm is fine too.

I know what RAID is :-)

Not really, disks are cheap enough that I'd rather keep things simple. I have redundancy in various ways in addition to using Backblaze.

Ports and physical space for drives aren't free. (Though there's some trade-off available between time/money if you want to DIY.)

For my own use, I've found it isn't worthwhile expanding beyond eight drives rather than getting rid of smaller ones - adding additional ports costs money and it takes more power and space.