Mastodawn

@lamp I recommend to "circle-swap" drives and always exchange the smallest for a bigger drive...

Ideally get the biggest #Nitrogen-Filled #CMR drive you can afford and setup a #ZFS #ZRAID2 or even better #ZRAID3 to get expanability...

@fuchsiii also you propably should consider using at least #DUP, #RAID6 / #ZRAID2 or #mdraid10 (in redundant striping configuration)...
https://en.wikipedia.org/wiki/Non-standard_RAID_levels#DUP_profile
https://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_6
https://en.wikipedia.org/wiki/ZFS#ZFS's_approach:_RAID-Z_and_mirroring
https://en.wikipedia.org/wiki/Linux_MD_RAID_10

Or a file-based (de)duplication approach as well as a database similar to #LTFS indexes and #TapeLibraries do.
https://en.wikipedia.org/wiki/Linear_Tape_File_System
https://en.wikipedia.org/wiki/Tape_library
https://en.wikipedia.org/wiki/Hierarchical_storage_management
https://en.wikipedia.org/wiki/Amanda_(software)

Non-standard RAID levels - Wikipedia