I have a few RAID 5 disks from a computer that broke and I’m wondering, I have them all plugged in but how do I reformat them for normal use? My Linux machine doesn’t even pick up the devices. Just a few gigs of TMPFS.
The disks should show as devices in your system but they most likely don't have any valid partition table on it.
This makes them unavailable for automatically mounting/showing them on your desktop.
gparted or any other partitioning tool of your choice.
It should show the disks as either raid-member or as unformatted.
Simply give it a fresh partition table (either MBR or GPT, depending on your system and BIOS, when in doubt use the same type as the disk your /-filesystem is on) and then you can proceed as usual creating partitions.